Radio Receivers and Servicing

sussawea®

t? Sees
e

oe

SC gra ee
tte wy
2 Ce

Radio Library
Vol. IV

Radio Receivers and
Servicing

RADIO RECEIVERS
By
K. M. MacILVAIN, E.E.

RADIO ENGINEER; MEMBER,
INSTITUTE OF RADIO ENGINEERS

SERVICING OF RADIO RECEIVERS
By
L.Gssic STAFF:

Published by
INTERNATIONAL TEXTBOOK COMPANY
SCRANTON, PA.

Copyright, 1928, by INTERNATIONAL TEXTBOOK COMPANY

Copyright in Great Britain

All rights reserved

Printed in U.S. A.

INTERNATIONAL TEXTBOOK PRESS
Seranton, Pa. 93898

PREFACE

The popular interest in radio is due chiefly to the
reception of programs transmitted by radio broadcasting
stations. An immense industry has been built around
this branch of radio, giving employment to thousands who
by their training and experience are qualified to serve it.

This volume was prepared especially to acquaint the
reader with the fundamentals of radio reception and
with methods of locating and overcoming the difficulties
in radio reception. The instruction on Radio Receivers
begins with the crystal detector and is followed logically
by regenerative receivers, radio-frequency amplifiers,
neutrodyne sets, reflex set, superheterodyne receivers,
short-wave receivers, single-side-band receiver, power
amplifiers, a.-c. receivers, and loud speakers.

The Section on Servicing of Radio Receivers contains
practical instructions for locating and remedying troubles
in radio receivers, loud speakers, power units, and acces-
sories.

This instruction will be helpful to the men in the
‘industry, such as operators, set builders, dealers, sales-
men, and service men. In fact, every set owner will profit
by this instruction inasmuch as it will acquaint him with
the possibilities and limitations of his own set and teach
him how to obtain the utmost service from the equipment
he may have.

INTERNATIONAL CORRESPONDENCE SCHOOLS

CONTENTS

RadiotRecervers 260 ie hes vee ss 60a 1S at A
Pundamental [Theory of Operation): os ieee ee ease es
Perey stall etecbOrs .. 40 cae hie RRC Um cae Urea aan Os

Macuitiinie 1 Ube. ELECUOIS «ec eee eee teeta rule 4

Diode detector; Bias detector; Detector using grid condenser and
grid leak; Reception of undamped waves; Interception and
detection.

Rerenerative Receivers. hee ls che eee

Single-circuit receivers; 300—-19,000-meter commercial receiver;
Regeneration by tuned-plate method.

eaciosrequency Am plihétss2 saa eee eed ee ce

Untuned-transformer coupled radio-frequency receiver; One-stage
tuned radio-frequency with feed-back; Two-stage tuned radio-
frequency receiver; Neutrodyne receiver; Reflex receiver;
Superheterodyne receiver; Short-wave receivers; Single-side-
band receiver.

Pucio-eraciency: Aimplifiers., «rc cur. ceisler ait

Types of audio-frequency amplifiers; Transformer-coupled audio-
frequency amplifier; Impedance-coupled audio-frequency
amplifier; ([ransformer-resistance coupled audio-frequency
amplifier. —

Power Amplifiers and Power Plate Supply..............

Advantages of power amplifiers; Power amplifier and power
supply with full-wave rectification; Power amplifier with B
and C eliminator; Receivers with a.-c. tubes.

RCMEELE TOC UCETS » scary (iiits 015 ter) arcu ete el sale! eden eR ass Ge
Telephone receivers; Speakers.

penvaicine OF Radio HeCelVerss ¢ oca-aicegh ee ee ae

Teremernl INStrucllONs v5 cee cc) ou dls cledaee 2 UAT ae ety in

Classification of receiving sets; Precautions; Service-shop equip-
ment; Portable tool kit; Serviceman’s conduct; Obtaining
information from customer; Relation between length of service
and failure. ,

rouse Dicsmourees.of LrOUDIO.. s« falc. -kiearee Ree

Trouble in accessories; Outside interference.

Seating OF RECEIVING ets vyacthe ae bd fe ee tee :

Weston a.-c. and d.-c. tester; Testing battomeoversred et
Testing a.-c. operated sets.

Specific Troubles and Adjustments................ :

Simple testing equipment; Variable-condenser troubles; esting
fixed condensers; ‘Adjustment of neutralizing condensers; Ser-
vicing of power units; Servicing of radio speakers.

14— 18

192569

L018

(hee dae

96-103

104-141
104-110

111-117

118-129

130-141

a
‘
hi
Be
»
¢
« i"
a?

RADIO RECEIVERS AND
SERVICING

RADIO RECEIVERS

FUNDAMENTAL THEORY OF OPERATION

In the course of radio reception, the receiving antenna
is subjected to the field of a traveling wave emanating
from a radio transmitting station, and radio-frequency
currents are induced in the antenna system. This radio-
frequency energy is very feeble and some appreciation of
this fact may be derived from the following discussion.

A non-directive radio transmitting station will be con-
sidered. Since it is not directive,.energy is radiated from
this transmitter with equal strength in all directions. Ata
given distance from the transmitter the energy radiated
is scattered over the entire surface of a sphere having a
radius corresponding to the distance from the transmitter.
Owing to the relative size of the receiving antenna, the
latter can only cover an extremely small fraction of the
sphere in question, hence the minuteness of the induced
currents.

At the receiving station the antenna functions to inter-
cept the traveling waves from the transmitting station
and the action is manifested by the very feeble radio-
frequency currents in the antenna circuit. The problem
at this point is to establish a means of sensing the inter-
ception of radio waves. In regard to the feasibility of
visualizing the current in the antenna circuit, it should be

2 SR ADIO SRBC EDV ERs

considered that a radio-frequency milliammeter would
not be sufficiently sensitive to indicate the value of the
eurrent.' Even if it could, it would have too slow an
action to follow the dots and dashes of the telegraph code
at the speed with which they are transmitted in normal
operation. In the case of radio telegraph communication,
if a meter of sufficient sensitivity could be produced
whose indicating element could follow the dots and dashes
of the code, the signals could be read by the eye, but this
is extremely impractical and it has been found that
aural reception of radio telegraph signals must first be
effected.

It might be stated at this point that in large commercial
radio telegraph receiving stations these received audio-
frequency signals are ampli-
fied and then rectified, pro-
ducing dots and dashes in
Se ee the form of unidirectional

g
e (j pulses. These pulses are
applied to sensitive re-
corders that record the dots

; and dashes in ink on a moy-
ing tape. This makes it possible for the receiving operator
to receive by either the eye or the ear or both.

In the case of radio telephone reception, visual reception
would be unintelligible, so this necessitates aural reception.
Thus in the case of radio telegraphy and radio telephony
aural reception is necessary. A fundamental receiving
circuit is shown in Fig. 1. The antenna a is connected
to one end of the coupling coil 6, and the ground ¢ is
connected to the other end. The radio-frequency currents
induced in the antenna pass to ground through the coil b.
The antenna circuit is not tuned to any particular wave-
length, hence it is considered aperiodic.

= Fic. 1

AN DOGS@RVICIN Gr: 3

The two coils 6 and d constitute a radio-frequency step-
-up transformer. ‘This is desirable, owing to the fact that
the feeble energy in the antenna circuit consists of a rela-
tively high current and low potential, and what is desired
for application to the detector is a low current at a high
potential. Thus, the potential available across the coil b
is stepped up to a relatively high potential, which is
available across the coil d. This potential is still further
increased by shunting a condenser e across the coil d so
that the combination may be tuned to the frequency of the
incoming wave. This tuning operation not only produces
maximum voltage cross the coil d and the condenser e at
the desired wavelength, but it also selects the wavelength
desired and tends to suppress the application of signal
voltages on other wavelengths to the detector circuit.

The function of the detector f is to change the radio-
frequency signal into an audio-frequency signal that can
be applied to the phones g. The thought arises at this
time, why not connect the phones directly in series with the
antenna circuit. If there is enough energy in the output
circuit of the receiver shown in Fig. 1 to actuate the dia-
phragm of the telephone receivers, or phones, g there
should be sufficient energy to accomplish this operation in
the antenna circuit. <A consideration of Fig. 2 will help
explain why the phones cannot be made to function in the
antenna circuit.

A series of wave trains that are sent out from a damped-
wave radio-telegraph transmitting station, such as a
quenched-spark transmitter, are shown in Fig. 2 (a).
Each time the transmitting key is pressed down, wave
trains are sent out at an audio-frequency rate, possibly
1,000 per second. Each one of these wave trains is made
up of radio-frequency oscillations that are of the order of
500,000 cycles per second, if the wavelength is 600 meters.

4 SRA D 10) RIEC. BA Ven Ris

Even if there were sufficient. current in the antenna
circuit, caused by the incoming signal, to operate the
receiver diaphragm, the diaphragm could not follow the
radio-frequency changes in current because it has a period
of its own and also possesses a certain inertia which pre-
vents it from vibrating at such a high rate. Even if it

kadio -Frequencty
(pur

Rectified
Current

N
x
&
&§ +
G8
x
© 9
Ow
LES
e
>
ge oO
(©
inde, &

could vibrate at such a high rate, the human ear would not
be affected, since the highest frequencies audible to the
human ear are between 16,000 and 20,000 cycles per second.

It might then be reasoned, why does not the telephone
receiver connected in series with the antenna circuit follow
the average change in current. The answer is, it does.

AND SERVICING 5

The average change in current is zero, as will be noted
from a consideration of Fig. 2 (a). The radio-frequency
variations in current go as far in the positive direction as
in the negative direction and the average is zero. The
function of a detector is to change the nature of the radio-
frequency in such a manner that its average value will not
be zero, and it does this by rectifying the radio-frequency
input as shown in view (0b).

The detector is a device that has unilateral conductivity.
It allows the passage of current in one direction only.
The natureof the current in the output circuit of the
detector is shown by view (6). The detector suppresses
the negative waves, and the average of the positive waves
results in a positive current that is applied to the phones.
The telephone-diaphragm vibrations for each wave train
are indicated by view (c). The pulses of current indicated
in Fig. 2 occur at an audio-frequency rate, 1,000 per second,
hence the diaphragm in the telephone receivers can
respond to the current variations and vibrates in synchron-
ism with the current pulses. This vibration sets up sound
waves that are sensed by the ear. Thus, every time that
the transmitting key is depressed a 1,000-cycle note is
heard in the phones.

In the case of radio telephony, the radio-frequency
oscillations vary in accordance with the audio-frequency
signals which it is desired to transmit. The radio-fre-
quency input to the receiver is in the nature of a radio-
frequency current whose amplitude is varying at an audio-
frequency rate. The negative halves of the radio-fre-
quency oscillations are chopped off by the action of the
detector, and the phones record the audio-frequency varia-
tions in the current in the detector output circuit. The
term detector is misleading. This device is virtually a
rectifier and the phones detect the incoming signal.

6 SR ADIO SRECEIVERS

CRYSTAL DETECTORS

There are a number of crystals that have unilateral
conductivity; that is, they offer a high resistance to the
passage of a current In one direction and a low resistance to
the passage of current in the opposite direction. The
following is a list of some of the crystals that have this
property: iron pyrite, galena, molybdenum, bornite, and
carborundum. A characteristic curve for crystals of this
type is shown in Fig. 3. It will be noted that the voltage
applied to the detector is plotted horizontally and the
resultant current through the detector is plotted vertically.

A consideration of the character-
istic crystal curve will reveal that,
if an incoming oscillation produces
approximately equal and opposite
potential variations across the
detector, the output current dur-
ing the negative half cycle is
negligible as compared with the
output current during the positive half of the cycle. This
effects detector action, or rectification.

Current

Fic. 3

VACUUM-TUBE DETECTORS

DIODE DETECTOR

A three-electrode vacuum tube may be used as a detector
or an amplifier or as a combination of both, according to
the method of making connections. When it is used as a
detector alone, it functions as a diode, or two-element tube,
and not as a triode, or three-element tube, so it follows that
a two-electrode vacuum tube can also be used as a detector.

The method of connecting a three-electrode vacuum
tube to make it function as a detector in a receiving circuit,
without using its amplifying propensities, is shown in

AND SERVICING a

Vig. 4. The vacuum tube with its plate and grid con-
nected together is inserted in place of the crystal detectro
as previously’ explained.
The plate-grid connection
forms one terminal of this
type of detector and the
negative filament connec-
tion is the other detector
terminal. In the course of
broadcast reception, there
is small chance of distor-
tion occuring in the detector circuit when this scheme of
connections is used.

¢—~

Fic. 4

BIAS DETECTOR

The scheme of connections for the bias detector are
shown in Fig. 5, and the characteristic plate-current
grid-voltage curve for a three-element tube is shown in
Fig. 6. The grid of the tube ts held sufficiently negative
by means of the bias, or C, battery, Fig. 5, to cause the
incoming signal voltage to operate on the lower bend of
the plate-current curve as shown in Fig. 6. The operat-
ing point on the plate-current curve is such that the
negative half of an incoming potential oscillation causes
far less change in the
plate current than the
positive half of the
oscillation. A train
of oscillations applied
to the grid of this type
of detector tube causes
an average change of
plate current which is
positive, hence detector action and amplification are both
effected. One of the advantages of this type of detector

Fie. 5

8 SR ADIO CRECEIVERS

is experienced in the course of radio broadcast reception.

The output of this type of detector is quite free from dis-

tortion caused by overloading,

because the grid of the tube is

held negative. Detector ac-

Alate-current tion takes place by virtue of

the fact that the negative half

of the incoming potential cycle

operates from the lower bend

Vga Brae of the plate current curve

downwards, and the positive

half of the incoming oscilla-

Grid Voltage tion operates from the lower

ae bend upwards. Thus, it would

be necessary for the grid voltage to be such as to carry

the plate current to a value on the upper end of the
characteristic curve to produce distortion.

Another advantage of this type of detector over the
diode type is that in the diode the unilateral impedance
characteristic of the tube is the only feature that is made
use of, whereas, in this case, its ability to amplify is made
use of and the incoming signal voltage is applied to the
grid of the tube; thus, its effect is multiplied in the plate
circuit by the amplifica-
tion factor of the tube.

Plate Current

C-Battery Voltage

DETECTOR USING GRID CON-
DENSER AND GRID LEAK

A schematic circuit
arrangement for effect-

. -A +B
ing detector action by -B

using a grid leak a and Fig. 7

grid condenser b is shown in Fig. 7. The action is shown
eraphically in Fig. 8. The incoming signal voltage is

» a
— =|

AND SERVICING 9

applied to the grid of the tube c, Fig. 7, and the grid is
alternately positive and negative. When the grid goes
positive, it not only causes an increase in current to the
plate of the tube, but the grid itself accumulates some of
the electrons that are flowing from the filament toward
the plate. When the grid goes negative it does not lose

+
Time ——-
QS 0 :
*®
ss |
Q'S |
$o_
|
| -
SO
i
3
Q -
S sh Time required for
§ Grid Charge to Leak off

+

Lp

Plate Current

Ss

Time ——»

Motion of
Feceiver

Time ——>

Ide, ots

all of the electrons that it has accumulated, owing to the
fact that it takes longer for any appreciable amount of
electrons of leak off, than the time duration of the negative
half of the cycle of the oscillatory signal input, this being
a function of the value of grid leak used. Thus, before the
electrons leak off from the grid, the latter goes positive,

IO CR ADIO CRB GE) Livan iRes

again and accumulates more electrons. It is in this man-
ner that the grid gradually becomes more and more
negative during the passage of a wave train. The sub-
sequent effect is to cause an average change in the plate
current that is less than normal and it is in this manner
that detector action is effected in this type of detector.
The function of the grid leak a is to allow the negative
charge on the grid to leak off between wave trains and,
to prevent the electrons from leaking off between oscilla-
tions.
RECEPTION OF UNDAMPED WAVES

The fundamental circuits discussed have all been for the
reception of waves whose amplitude changes at an audio-
frequency rate. For instance, in the case of the signals
from a quenched-spark transmitter, the wave-train fre-
quency is, say, 1,000 cycles per second; therefore, the
amplitude of the radio-frequency waves reaches its maxi-
mum value and its minimum value 1,000 times per second,
and it is by virtue of this fact that the detector is able to
produce the desired sound which is heard in the ear
phones.

In the case of icw. (interrupted continuous wave), the
continuous waves generated at the transmitter are cut in
and out at an audio-frequency rate by means of a chopper.
If the chopper turns the radio-frequency oscillations of
continuous amplitude on and off 1,000 times per second,
the amplitude of the transmitted wave will reach its
maximum and minimum 1,000 times per second. It is
by virtue of this fact that detector action at the receiver
produces audio-frequency sounds through the medium of
the ear phones.

In the case of radio-telephone transmission and recep-
tion, the amplitude of the radio-frequency oscillations
generated varies at an audio frequency rate according to

AND SERVICING II

the frequency of the speech or music that it is desired to
transmit. It is by virtue of this fact that detector action
changes the modulated radio-frequency input into audio-
frequency currents.

The reception of undamped, or continuous, waves differs
somewhat from the foregoing. An undamped wave is one

+

(a)

(d)

(e)
Fic. 9

whose amplitude remains constant and it is necessary that
the amplitude of the radio-frequency signal applied to the
detector should vary at an audio-frequency rate in order
that the detector can function. To change the input into
the audio-frequency desired, in the case of ew. it is neces-
sary to provide means at the receiver of changing the
amplitude of the radio-frequency input at an audio-
4—2

12 SR ADIO SRECEIVERS

frequency rate so that its presence may be manifested by
sounds in the ear phones.

In Fig. 9 (a) is shown the nature of the incoming ew.
signal that is applied to the detector in the circuit shown
in Fig. 10, which is a schematic wiring diagram of a cir-
cuit for the reception of undamped-wave signals. The
traveling wave is intercepted by the antenna. The
induced currents in the antenna system pass through the
antenna coil a, setting up a magnetic field around this
coil. This magnetic field threads through the coil 5b,
inducing currents therein of the same frequency as the
induced currents in the antenna circuit. Owing to the

fact that coil 6 and con-

denser ¢ are tuned to the

same frequency as that
a

of the incoming wave,
there will be maximum
voltage across the coil b
and the condenser c.
There is a local genera-
tor d of radio-frequency
oscillations having a frequency 1,000 cycles greater (or
less) than that of the incoming signal. Energy at this
frequency is induced into the detector circuit by means of
the coupling coils e and f. The current generated by
the local oascillator is represented in view (6b), Fig. 9.
Thus these two frequencies are superimposed one upon
the other and there is a resultant beat frequency, shown in
view c, which is equal to the difference between the two
radio frequencies, or 1,000 cycles. The amplitudes of
views (a) and (b) are simply added for each interval of
time, and the result, as shown in view (c), is an alternat-
ing current of periodically increasing and decreasing
amplitude, the alternating current being at radio fre-

isis, 1G

AND SER VIECING 13

quency, and the rate of change of its amplitude, from maxi-
mum to minimum, being at audio frequency. The nature
of the current in the output circuit of the detector is shown
in view (d). The negative half of each radio-frequency
oscillation is chopped off; hence, the average change in
the rectified current occurs at an audio-frequency rate,
and it is this audio-frequency change in the rectified cur-
rent, shown in view (e), that actuates the diaphragm of
the telephone receivers.

Considering Fig. 10, a vacuum tube could be used at d to
generate the radio-frequency oscillations and a crystal
detector at g. Again, a single three-electrode tube could
be used in a circuit to function as a detector, an amplifier,
and an oscillator, as will be explained later.

INTERCEPTION AND DETECTION

From the foregoing discussion it is found that the recep-
tion of radio signals is fundamentally a case of intercep-
tion and detection. ‘There must be a means of intercept-
ing the electro-magnetic waves travelling through the
ether and a means of changing the radio-frequency cur-
rents induced in the antenna system into audio-frequency
currents so that they may in turn be changed into sounds
_of audio frequency intelligible to the human ear. The
crystal detector changes the radio-frequency current into
an audio-frequency current and the telephone receivers
effect the change from audio-frequency currents to audio-
frequency sound waves.

Greater sensitivity, or reception from a greater dis-
tance, is effected by increasing the amount of signal
energy applied to the detector. This can be accomplished
by increasing the efficiency of the antenna system or by
amplifying the radio-frequency input before application
to the detector.

14 | SRADIO “RECEIVERS

Greater output volume with a given amount of signal
energy available in the detector output circuit is a func-
tion of the amount of audio-frequency amplification
effected.

The fundamental elements involved in radio reception
have now been considered. The next consideration will
be the different types of receivers, so that it may be learned
how the fundamental elements are embodied in the various
receivers designed for different wavelengths and duties.

REGENERATIVE RECEIVERS
SINGLE-CIRCUIT RECEIVER

The schematic wiring diagram of a single-circuit receiver
is shown in Fig. 11. The detector input circuit is tuned
by means of the inductance coil a and the condenser 6.
The antenna lead is connected directly to the grid input
coil a, and the ground is connected directly to the low end
of this coil. The grounded end is also connected to the
rotor plates of the tuning condenser 6.

A grid condenser c and
grid leak d are connected
in series with the grid
lead to effect detector
action. The grid return ,
is connected to the nega-
tive filament terminal.
This is the scheme of
connections when using a

= Ata +8 UX-200-A detector tube.
> Fic. 11 8B :

With some other types of
tubes slightly better results are obtained by bringing the
erid return to the positive filament terminal.

The plate of the tube is connected to the positive
B-battery terminal through the feed-back coil e and the

AND SERVICING 15

phones f. It is by means of the inductive relation between
the feed-back coil e and the input coil a that regeneration
is effected. Regeneration is the feeding back of the radio-
frequency signal energy from the plate circuit to the grid
circuit, thus allowing it to be reamplified, or boosted,
again. This regenerative effect may be carried to the
point of oscillation. ‘This point is where the tube starts
to oscillate, thus generating oscillations of continuous
amplitude and of a frequency that is determined by the
constants of the tuned input circuit, consisting of the coil a
and the condenser 0.

A circuit of this type can be used to receive damped or
undamped radio telegraph signals or it can be used for
radio telephone reception. With all the elaborate receiv-
ing circuits that are in existence today, there are many
that can not equal the performance of this little single-
circuit receiver that was one of the first types of broadcast
receivers to appear.

300- TO 19,000-METER COMMERCIAL RECEIVER

A schematic wiring diagram of a standard commercial
_ receiver that is used on ships for the reception of telegraph
and telephone signals on wavelengths between 300 and
19,000 meters is shown in Fig. 12. The receiver proper
* provides adequate switching arrangement for covering all
wavelengths between 300 and 8,000 meters, employing
either a crystal detector or a vacuum-tube regenerative
detector. There is a long-wave attachment for this set
that allows for tuning in signals on wavelengths as high as
19,000 meters. There is also a two-step amplifier attach-
ment for increasing the output volume of the received
signals.

The antenna is connected to a contact arm that can be
moved to different taps on the primary winding a. The

CR ADIO SRECEIVERS

16

ACN) RV. CLIN, G 17

low end of this winding is connected to the stator plates ofa
.00045-microfarad variable tuning condenser b through two
external terminals c. These two external terminals are
jumpered together when the receiver is being used on the
300- to 8,000-meter band, but when it is desired to tune in
stations between 8,000 and 19,000 meters a primary loading
coil is inserted at this point in the circuit. The rotor plates
of the primary tuning condenser b are connected to ground.

The secondary circuit is coupled to the primary circuit
by means of the coupling coil d, which is inductively
coupled to coila. Other than the coupling just mentioned,
there is no coupling between the primary and secondary
circuits. A shield e is inserted between the two circuits.

The .00032-microfarad secondary tuning condenser f is
shunted across the three coils d, g, and h. Coil d is the
coupling coil between the primary and secondary circuits;
coil g is the coupling coil between the plate and grid of the
detector tube (when such is used) to effect regeneration;
and coil A is the tapped secondary tuning coil. Two
external terminals 7 are connected in series with the
secondary inductance coils to allow for the insertion of a
secondary loading inductance for tuning above 8,000
meters.

The stator plates of the secondary tuning condenser f
are connected to one of the poles of a four-pole double-
throw switch, which is used to change from a crystal
detector to a vacuum-tube detector. The two positions
of this switch may be designated by T and C, T being the
tube position and C the crystal position. When the
switch is in the tube position, the high side (stator plates)
of the condenser f is connected to the grid of the tube
through the grid leak and grid condenser unit. When the
switch is in the crystal position, the high side of the con-
denser is connected to one terminal of the crystal detector.

18 CR ADIO SRECEIVERS

The plate of the vacuum tube is connected to the feed-
back coils 7, the two external terminals k for the long-wave
tickler, and the contacts of the change-over switch. These
contacts connect the reactance I to the plate of the tube
in the tube position, and to the second terminal of the
crystal detector in the crystal position.

This reactance J is tuned by means of the condensers m
and n to the frequency of the signal energy in the output
of the detector (audio frequency). The other end of this
reactance J is connected through the phones o to the
positive B-battery terminal or to the rotor plates of the
secondary tuning condenser f, according to whether the
change-over switch is in the tube or the crystal position.
One of the filament leads to the vacuum tube passes
through the contacts of the change-over switch, so that
the tube filament is not energized when the crystal
detector is being used. A push button p is provided,
which, when depressed, short-circuits the plate coil 7.
This is known as the oscillation test.

This type of receiver is to be found on the majority of
ships at sea at the present time. ‘There may come a time
within the next few years when the same receiver will be
used with a stage or two of radio-frequency amplification
ahead of it, but results obtained with this set are at pres-
sent of such a high standard that it will be some time
before it will be superseded by a later model.

REGENERATION BY TUNED-PLATE METHOD

In Fig. 13 is shown a method of effecting regeneration
without establishing inductive coupling between the plate
and the grid coils. The antenna circuit is aperiodic
(untuned), and the grid input circuit is tuned to the incom-
ing signal by means of the inductance coil a and the
condenser b.

AN DOSER V UCINIG 19

Regeneration is effected by tuning the plate circuit to
the incoming signal by means of the inductance coil ¢ and
the condenser d. The feed-back from the plate to the grid
circuit of the tube is effected by virtue of the capacity
coupling between the grid and the plate that is inherent
within the tube itself.

RADIO-FREQUENCY AMPLIFIERS

UNTUNED-TRANSFORMER COUPLED RADIO-FREQUENCY RECEIVER

A schematic wiring diagram of a receiver that has
three stages of radio-frequency amplification ahead of the
detector is shown in Fig. 14. This receiver employs

-A +A +B
-B

reels

fl

untuned transformers between the stages of radio-fre-
quency amplification. ‘The antenna is connected to one
side of the primary winding of the first radio-frequency
transformer a and the ground lead is connected to the
other end of the same winding. One side of the secondary
winding is connected to the grid terminal of the first radio-
frequency amplifier tube b and the other end of the same
winding is connected to the movable contact arm of the
400-ohm stabilizing potentiometer c. The extremities of
this potentiometer are connected across the A-battery
supply leads.

20

ms!
() ()

SRADIO “RECEIVERS

Fic. 14

AND SERVICING 21

The two remaining radio-frequency transformers d and
e have their primary windings connected in series with the
plates of the two radio-frequency amplifier tubes 6 and f,
respectively, and their secondary windings in series with
the grid circuits of the two radio-frequency amplifier
tubes f and g, respectively.

The low side of the secondary winding of each of the
three radio-frequency transformers is connected to the
movable contact arm of the stabilizing potentiometer c.
The function of the stabilizing potentiometer is to afford a
means of supplying a small positive potential to the grids
of the three tubes in question, which tends to keep them
from oscillating.

There is no frequency selection ahead of the detector
tube in a receiving circuit of this type. All energy induced
in the receiving antenna is passed on to the first radio-
frequency amplifier tube and thence to the two succeeding
stages, where all incoming radio signals are boosted in
voltage for application to the detector tube. The function
of the radio-frequency amplifier system in this circuit is
to step up the voltage of all the radio-frequency signals
that reach it through the medium of the antenna.

The input to the detector is tuned. It is here that a
selection is made of the particular signal that it is desired
to receive. ‘The tube g may be considered the output tube
of the radio-frequency amplifier. The output circuit of
this tube is coupled to the grid circuit of the detector tube
h through the plate coil 7 and the grid coil 7. The detector
input circuit is made selective by means of the tuned cir-
cuit consisting of the coil 7 and the condenser k. The
wavelength range depends on the values of the inductance
and capacity of these devices.

Regeneration is effected in this circuit by means of the
coil / in series with the detector plate circuit and coupled

24) ‘RADIO SRECEIVERS

to the detector input circuit by means of the inductive
relation between the coils 7 and l. The condenser m is a
radio-frequency by-pass condenser, which functions to
by-pass the radio-frequency currents in the plate circuit
of the detector tube around the phones n and the B bat-
tery, as they offer a relatively high impedance to the pas-
sage of currents at radio frequencies. The impedance of
the path for radio-frequency currents in the plate circuit
of a regenerative detector tube should be as low as possible
so as to effect regeneration, if desirable, to a value just
below the oscillating point.

ONE-STAGE TUNED RADIO-FREQUENCY WITH FEED-BACK

Circuit Connections.—It was previously pointed out
that the old single-circuit receiver with regeneration was
one that would offer good competition to many of the
elaborate receivers that have been produced since the
inception of radio broadcasting. A modification of this
receiver is shown in Fig. 15. There is one stage of tuned
radio-frequency amplification which boosts the signal
voltage before application to the detector tube and also
selects the frequencies desired. Regeneration is effected
by a feed-back from the plate circuit of the radio-fre-
quency amplifier tube to the antenna circuit by means of
the coupler a.

The grid circuit of the radio-frequency stage is really
a radio-frequency filter. It effects the greatest voltage for
application to the grid of the radio-frequency amplifier
tube at that frequency to which it is tuned. I¢ will also
pass frequencies several thousand cycles greater and
several thousand cycles less than that frequency to which
it is tuned, but with less and less efficiency, depending on
the number of cycles difference between the frequency in
question and the fundamental frequency and on the sharp-

AND SERVICING 23

ness of tuning of the circuit containing the inductance coil
b and the condenser c. This sharpness of tuning is a
funetion of the amount of resistance in the tuned circuit;
the less the resistance the sharper the tuning and the
greater the resistance the broader the tuning.

In the course of radio broadcast reception the receiving
antenna is subjected to the field of a traveling wave con-
sisting of a carrier frequency with side bands usually up
to 5,000 or 10,000 cycles on either side of the carrier.
The carrier is the radio frequency that is generated by the

Bias

Fre. 15

apparatus in the transmitter and upon which the audio
frequencies that are to be transmitted are superimposed.
The audio frequencies that are usually transmitted in the
course of a radio broadcast are those up to 5,000 cycles.
The superimposition of this 5,000-cycle audio-frequency
band on the carrier effects the transmission of what is
termed the upper side band, with limits of the carrier
frequency and the carrier frequency plus 5,000, and the
transmission of what is termed the lower side band, with
limits of the carrier frequency and the carrier frequency
minus 5,000 cycles. ‘Thus it can be seen that it is necessary
to pass a 10,000-cycle band through the tuning circuits.

24 FRADIO SRECEIVERS

It is very fine to have sharpness of tuning, or selectivity,
which means the passage of a narrow band of frequencies,
but it is not desirable to have too great a degree of selec-
tivity, as this would mean that some of the frequencies in the
side bands would be chopped off and distortion would ensue.
In order to effect undistorted reception the loud speaker
must reproduce all the audio frequencies that are trans-
mitted by the broadcasting station, it being assumed that
the broadcasting station isputting out anundistorted signal.

List and Description of Parts.—It is important in con-
sidering the construction of radio broadcast receivers to
obtain the best apparatus.
If inferior apparatus is used,

Sy
Lf

Gh

Fic. 16 Fic. 17

it may work well for a short time, but there is no assurance
that satisfaction will be long-lived. In Fig. 15 the follow-
ing apparatus is required:

a—Variocoupler. This device consists of two coils, a
stator a, Fig. 16, and a rotor b. Coil a is wound with
30 turns No. 24 d.c.c. (double-cotton covered) wire on a
3-inch form. Coil b is wound with 30 turns No. 24 d.e.e.
on a form that is free to turn within the 3-inch form of
coil a.

b—Inductance coil, Fig. 15. Thisis a 44-turn spiderweb
coil tapped at fourth turn for the antenna connection,
and wound with No. 24 d.c.c. on a 2-inch form, as shown
in Fig. 17.

AND |) SERVICING 25

ec and d—Variable condensers, Fig. 15, .00035 micro-
farad.

e—Grid condenser, fixed, .00025 microfarad.

f—By-pass condenser, fixed, .1 microfarad.

g—By-pass condenser, fixed, .002 microfarad.

h—Radio-frequency transformer. ‘The secondary wind-
ing s is a 44-turn spider-web coil, No. 24 d.c.c. on a 2-inch
form. The primary winding 7: consists of 6 turns, No. 24
d.c.c. wound on the outside of the secondary coil.

.2 and j—10-ohm rheostats.

k—400-ohm potentiometer.

l—Grid leak, 3 megohms.

m—Vacuum tube, UX-201-A and socket.

n—Vacuum tube, UX-200-A and socket.

In addition to the foregoing, it will be necessary to have
a panel; a base board; about 15 feet of bus wire; control
knobs for the variable condensers, rheostats, potentio-
meter, and variocoupler; the required A and B batteries,
(A, 6 volts, B 90 volts, tapped at center for detector-plate
connection) ; and a pair of telephone receivers (2,000 ohms).
If a two-stage audio-frequency amplifier is used in con-
junction with this receiver, it is permissible to have the
radio- and audio-amplifier tube filament temperature
controlled by the same rheostat.

TWO-STAGE TUNED RADIO-FREQUENCY RECEIVER

Circuit Diagram and List of Parts.—The one-step tuned
radio-frequency receiver is the first step beyond the single-
circuit tuner, and the two-step tuned radio-frequency
receiver 1s the next step beyond the former. It is quite
easy to construct a receiver having a single stage of radio-
frequency amplification that will operate with satisfactory
stability, but it is not so easy to effect stable operation
with two stages of radio-frequency amplification, because

26 (RADIO SRECEIVERS

the inductive and capacitive feed-backs between the radio-
frequency stages tend to cause the radio-frequency
amplifier tubes to oscillate.

The inductive feed-back is caused mostly by the inter-
linking of flux from the tuning coils in the radio-frequency
amplifier stages, and the capacity feed-back is caused
mostly by the inherent electrode capacity within the
tubes themselves. In this receiver the inductive coupling
between successive radio-frequency stages has been
minimized by the use of closed field coils. These coils are
termed D-coils, or figure-8 coils.

A schematic wiring diagram of the receiver under
consideration is shown in Fig. 18. The actual apparatus
used in the construction of this set is shown in Fig. 19.
The following is a list of the material.

a—D-Coil, 3-inch diameter with 14-turn primary and
56-turn secondary.

b—D-Coil 3-inch diameter with single 56-turn winding
(tapped at turn 14).

c—D-Coil 3-inch diameter with 14-turn primary and
56-turn secondary.

d—<Audio-frequency transformer (6 to 1).

e—Audio-frequency transformer (2 to 1).

f—Output transformer (1 to 1).

g—.0005-microfarad variable condenser.

h—.0005-microfarad variable condenser. .

i—.00025-microfarad variable condenser.

j—.0005-microfarad variable condenser.
k—.00025-microfarad grid condenser.

I—.002-microfarad by-pass condenser.

m—Four .1-microfarad by-pass condensers.

n—200-ohm potentiometer.

o—6-ohm rheostat.

p—10-ohm rheostat.

27

ol SERVICING

Ob -
INES ogee
G29 +

O06 +

O'SE/
ON ie

= ail

SI “OIA
lasts as eR) oe eee
Jseee =)

tla | | 4

ib

Se

io)

TE Q0Q0)

WOOOD

~

Am

28

CR ADIO ‘RECEIVERS

Bos

L

AND SERVICING 29

g—5-megohm grid leak.

r—3 megohm grid leak.

s—Two double-circuit output jacks.

t—Single-circuit output jack.

u—Filament switch.

v and w—Two UX-201-A amplifier tubes and sockets.

x—U X-200-A detector tube and socket.

y—UX-201-A amplifier tube and socket.

_g—UX-171- power-amplifier tube and socket.

The above is a list of the material that was used by the
writer in the construction of a receiver to aid in the
description of the functioning of this particular type of set.

Construction of Radio-Frequency Transformers.—The
main feature in the receiver shown in Figs. 18 and 19 is
the type of radio-frequency transformer used to minimize
interstage inductive coupling, and, although this type of
coll has been used commercially for some time, the writer
had the honor of being the first to present them to the
broadeast public through the medium of radio magazines.
When the first receivers of this type were constructed
there were no coils on the market that were of the particular
type embodied in this set, so it was necessary for the set
builder to construct them himself. Therefore, the
construction of a radio-frequency transformer of the D-coil
or figure-8 type will be discussed and this discussion and
subsequent theory should give one a good idea of their
inherent characteristics.

The following is the description of the construction of
the D-coil: Procure a piece of bakelite tubing 3 inches in
diameter and 34 inches long. Cut a slit 4 inch wide
through the side of the tube extending from one end to
within 2 inch of the other end. Cut a similar slit directly
opposite. The transformers a, b, c, Fig 19, show the
physical characteristics of the D-coil. Four terminals,

hel

L----}-/fA ~~ t—--~--

= coimille
All wi

AINED |S eR EVIL CO LNG 31

which may be labeled 1, 2, 3, and 4, are located around the
end of the tube that is not slit. The reason for putting
them at this end is because there is space that is free from
the winding and this end is more solid, as there is no
slit in it. Two of the four binding posts on each trans-
former are shown in Fig. 19.

Use No. 24 d.c.c. copper wire. A half-pound spool
will have enough wire for all three transformers. When
preparing to wind either transformer a or c cut off 20 feet
of the wire from the spool for the primary coil. Fasten
one end of this primary wire to the inside of terminal 2.
Fasten one end of the wire left on the spool to the inside of
terminal 3. This wire will form the secondary coil.
Wind both of these wires together through a slit and first
around one half of the form, then through the opposite slit
and around the other half of the form. This is continued
until 14 complete turns have been wound on the form,
whereupon the free end of the primary winding is brought
to terminal 1 and connected thereto. The secondary wind-
ing is continued until 56 turns have been wound. ‘The free
end of the secondary winding is then connected to termi-
nal4. The terminals /, 2, 3, and 4 of transformers a and c,
Fig. 18, are connected in the circuit as indicated in the figure.

The coil 6, Fig. 19, is wound in a manner similar to
coils a and c, except there is only one winding made up of 56
turns of No. 24 d.c.c. Coil bis tapped at the fourteenth turn
and from there is connected to the plate of the tube v, Fig. 18.

Radio-Frequency Receivers With Figure-8 Coils.—A
circuit digram of a receiving set with two stages of radio-
frequency amplification, a detector, and three stages of
transformer and choke-coil coupled audio-frequency
amplification is shown in Fig. 20. The construction of
this set is shown in Fig. 21. The following is a list of
parts indicated in Fig. 20.

a2 “RADIO

Fic. 21

SRECEIVERS

a—Feed-back coupler.

b—Three figure-8 radio-
frequency transformers.

c—A u dio - frequency
transformer (6 to 1).

d—200-henry impedance.

é—Double - impedance
coupler.

f—Speaker filter.

g—Three .00035-micro-
farad straight - line fre-
quency condensers.

h—.00025 - microfarad
grid condenser.

a—Three .05-microfarad
fixed condensers.

j—Two .1-microfarad by-
pass condensers.

k—.002 - microfarad by -
pass condenser.

l—Three .00025-micro-
farad fixed condensers.

m—200-ohm potentio-
meter.

n—'T wo 6-ohm rheostats.

o—10-ohm rheostat.

p—s-megohm grid leak.

g—500,000-ohm poten-
tiometer.

r—100,000-ohm grid leak.

s—single-circuit output
jack.

—Filament switch.

u—6-point switch.

AND SERVICING ae]

v—Two UX-201-A amplifier tubes and sockets.

w—UX-200-A detector tube and socket.

x—-Two UX-201-A amplifier tubes and sockets.

y—UX-171 power amplifier tube and socket.

Since the main feature of the receiver shown in Fig. 20
as well as of that shown in Fig. 18 is its ability to minimize
inductive interstage coupling, it will be well to find the
reason for this effect. The drawing shown in Fig. 22 will
aid in the explanation of the theory of the closed-field coil
in question. This theory applies to both the double-D
and the figure-8 coils.

A current through the
secondary winding of this
transformer passes in one
direction through the wind-
ing on the side marked a
and in the opposite direc-
tion through the winding
on the side marked b. The
lines of force emanating
from section a and those emanating from section 6 are in
opposite directions. These lines are additive through the
centers of section a and 6. The effects of the stray lines
of force in opposite directions from sections a and 6, in
the area surrounding the coil, have a tendency to neu-
tralize, as may be observed from the arrowheads on the
lines representing the lines of force in Fig. 22.

The flux density, caused by the current through the wind-
ings of the sections a and ), is greatest through the centers
of the two sections and is a minimum around the outside of
the coil. This is the reason why this type of coil is termed
a close-field coil. The field is closed through the center of
the two sections. ‘The foregoing discussion also shows why
the external field is a minimum.

j2iek ee

34. FRADIO SRECEIVERS

NEUTRODYNE RECEIVER

Neutrodyne-Receiver Theory.—If efficient and stable
operation in a radio-frequency amplifier system is desired,
it is necessary to eliminate both the inductive and capaci-
tive feed-back. The D-coil or figure-8 coil type of receiver
shows one method of eliminating, to a great extent, the
inductive feed-back. In the neutrodyne receiver a
method of neutralizing the capacity feed-back is put into
practice.

Thus, with the inception of capacity neutralization for
receiving sets it became possible to eliminate both the
inductive and the capacitive feed-back. A combination
figure-8 coil receiver with capacity neutralization con-
stitutes a decidedly worthwhile receiver. In the majority
of the standard neutrodyne receivers, which feature
capacity neutralization, inductive coupling is minimized
by setting the coils at a definite angle to each other. The
position of the coils causes the lines of force emanating
from one coil to pass through the other coils in a direction
parallel to the wires that constitute the winding of the
coil through which the flux lines are passing. As long as
the lines of force from one coil remain parallel to the wires
in the winding of a second coil, there will be no flux inter-
linkage and the inductive coupling will be zero.

A schematic wiring diagram of a section of a radio-fre-
quency amplifier is shown in Fig. 23, the radio-frequency
amplifier tubes being shown at a and b. The input circuit
of tube a is tuned to the frequency of the incoming signal
by means of the inductance coil ¢ and the condenser d.
The condenser e, shown by dotted lines, represents the
internal plate-grid capacity of the tube a. The output
circuit of tube a is tuned to the incoming signal by virtue
of the close coupling between the coils f and g, the latter

AND SERVICING 35

coil in parallel with the condenser h being tuned to the
same frequency as the combination of coil ¢ and con-
denser d. The condenser 72 is a by-pass condenser for
radio-frequency current from the positive B-battery
terminal to the negative A-battery terminal.

The maximum signal voltage in the output circuit of
tube a can be considered as existing across the inductance
coil f in view of the fact that the lower end of the coil
marked 2 is practically at ground potential, owing to the
radio-frequency by-pass condenser 7, which is of sufficiently
large value to offer very little impedance to the passage of
radio-frequency currents.

'

'
«.:
e
<
Pid

‘

i}

1

This maximum signal voltage that exists across the coil f
also exists across the internal plate-grid capacity of the
tube, represented at e, and the effective resistance of the
erid-filament circuit, represented at 7. The two quantities
in question are connected in series from the upper and
lower terminals of coil f, shown at / and 2. The reason
why the grid to filament circuit can be considered an effec-
tive resistance j is due to the fact that the voltage that is
considered is the voltage that is applied across the coil f,
and the frequency of this voltage is the frequency to
which the combination cd is tuned. Therefore, for this
frequency (the resonant frequency), the capacity reac-

36 FRADIO “RECEIVERS

tance and the inductive reactance of the circuit cd neu-
tralize, and the resistance of the circuit is all that is
left to impede the passage of current at its resonant
frequency.

If the frequency of the voltage across the coil f were
greater than the resonant frequency of the circuit cd, the
latter would be an effective capacity, and if the frequency
of the voltage across the coil f were less than the resonant
frequency of the circuit cd, the latter would be an effec-
tive inductance.

When the capacity e is quite small, its reactance is quite
large, capacity reactance being expressed by the formula

r
c

=——— ohms
2nfC
in which X,.=capacity reactance, in ohms;
a=constant 3.1416;
f=frequency, in cycles per second;
C'=capacity, in farads.

From the foregoing it can be seen that the potential at
the grid terminal of tube a, due to the voltage across
coil f, is above that of the ground by virtue of the cur-
rent from filament to grid, but is nearer the lower end 2
of coil f, owing to the fact that the voltage drop across
condenser e is much greater than the drop from grid to
filament.

If at any instant the polarity at point / is positive, then
the polarity at point 2 is negative, the two points being
180° out of phase. The grid, being nearer to point 2 than
to point 7, will be negative when point / is positive, and
this is the condition for regeneration, for here is a voltage
on the grid of the tube a that is of the same frequency as
the voltage in the plate circuit of the tube and, furthermore,
this voltage on the grid is negative when the voltage at the

AND» SERVICING 27

plate is positive. It is to be noted that this also is the
condition for self-oscillations: excitation voltage on the
grid, 180° out of phase with the plate voltage.

It might be interesting to note why the tube has a greater
tendency to oscillate on the lower wavelengths, during the
course of patrolling the broadcast wave band, than on the
higher waves. As the wavelength decreases the frequency
increases. As the frequency increases the reactance of the
internal plate-grid tube capacity decreases. Both of these
facts can be substantiated by considering the formula for
converting wavelength to frequency, in which

__ 800,000,000
~ wavelength

and the formula for capacity reactance, in which

1
on 2nfC

As the reactance of the tube capacity decreases the
voltage drop across it also decreases and the potential on
the grid becomes higher, hence more grid excitation, greater
regeneration, and, subsequently, greater tendency to oscil-
late.

One method of applying a neutralizing condenser to a
stage of radio-frequency amplification is shown at k.
The neutralizing scheme is virtuaily a wheatstone bridge.
The neutralizing condenser k is connected from the grid
terminal of tube 6 to the grid terminal of tube a. The
points 2 and 3 are at the same potential, so far as the high-
frequency currents are concerned, owing to the fact that
point 2 is a radio-frequency ground, through the medium
of the radio-frequency by-pass condenser 7, and point 3
is metallically connected to the negative filament lead,
which is at ground potential.

38 FRADIO SRECEIVERS

The signal voltage that is being fed back from the output
circuit of tube a can be considered, in this case, as existing
across the points / and 4 with the intermediate points 2
and 3 at ground potential. If the coil f is equal to coil g,
the point 2 or 3 is midway between the extremities / and 4;
hence if the grid is also made midway between / and 4,
as far as the voltage across the coils f and g is concerned, the
grid will be at ground potentials with respect to the feed-
back voltage, because the points 2 and 3 are at ground
potential. In this case, this can be accomplished by
making the neutralizing condenser k& equal to the plate-
grid capacity e.

In receiving tubes of the UX-201-A type the plate-
grid capacity is of the order of 6 micro-microfarads, thus
the neutralizing condenser k should have nearly the same
capacity. However, in most radio receivers employing
tuned radio-frequency amplification, there is more induc-
tance in the coil g than there is in the coil f. This is due
to the fact that there is a step-up ratio effected in the
coupling transformer between the output circuit of the
tube a and the input circuit of the tube b. The reason for
this is to boost the signal voltage available in the output
circuit of one tube, through the medium of the coupling
transformer, for application to the grid of a succeeding
tube.

If the inductance of the coil g is four times the inductance
of the coil f, four-fifths of the voltage drop from 1 to 4
will occur across coil g, and in order that condenser k
should function to maintain the grid at the same potential
as the points 2 and 3, there should be the same voltage
drop across it that there is across coil g. This can be
effected by making the value of condenser k one-fourth
that of condenser e, for the capacity reactance varies
inversely as the value of the capacity, the equation for

AN DS ER VLOUNG 39

capacity reactance being,

aE: |
+] +] #/ 7 +e h
Me= oO nfC whereas the
inductive reactance
XS varies directly as the

value of inductance, the
equation being, X,
Soom LA

Therefore, if the value
of the capacity of k is
one-fourth that of e, the
reactance of the former
will be four times greater
than that of the latter
and the voltage drop
across the former will
subsequently be four
times greater than the
voltage drop across the
latter. This is the con-
dition that must exist
to maintain the grid at
the same potential as
points 2 and 3, which
means that it is at ground
potential as far as the
feed-back voltage is con-
cerned. If condenser e
has a capacity value of
about 6 micro-micro-
farads, then 1.5 micro-
microfarads will be re-
quired at k for complete
neutralization.

Fic. 24

0000 ie

40 CRADIO °RECEIVERS

Standard Neutrodyne Receiver, 200 to 300 Meters.—A
schematic wiring diagram of a standard neutrodyne
receiver is shown in Fig. 24. The following is a list of the
receiving-circuit constants as well as a list of the material
needed in the construction of this set.

a—Three radio-frequency transformers, the primary of
which consists of 13 turns No. 24 d.s.c. (double-silk
covered) on 23-inch form and the secondary of 50 turns
No. 24 d.s.c. on the same form.

b—Three .0005-microfarad variable condensers.

c—6-ohm rheostat.

d—12-ohm rheostat.

e—3 megohm grid-leak resistance.

f—.00025-microfarad grid condenser.

g—.002-microfarad radio-frequency by-pass condenser.
h—.1-microfarad by-pass condenser

7—Two 1.5-micro-microfarad neutralizing condenser.

4—Two UX-201-A amplifier tubes and sockets.

k—U X-200-A detector tube and socket.

The antenna lead, Fig. 24, is connected to one end of fe
primary coil of the first PriNeirectt toe transformer, the
other end of which is connected to ground. The secondary
winding of this transformer is tuned by means of the .0005-
microfarad variable condenser 6. The ground lead is con-
nected through to the negative filament lead. The rheo-
stats are in the positive filament lead.

The primary winding of the second-radio-frequency
transformer is in series with the plate circuit to the first
radio-frequency amplifier tube. The secondary winding
of this transformer is also tuned by means of a .0005-micro-
farad variable condenser. |

The B-battery supply for the plates of the radio-fre-
quency amplifier tubes 7 should be between 67.5 and 90
volts.

AN De {SERV LCUN-G 41

The neutralizing condensers 7 are connected from grid to
grid. From a consideration of the preceding discussion of
neutrodyne theory the value of these condensers should
be of the order of 1.5 micro-microfarads, if the internal
plate-grid capacity of the tubes 7 is of the order of 6 micro-
microfarads. It is a good idea to use variable condensers
at 7 of such a maximum value that it is possible to pass
through the optimum point.

The solenoidal type of coils used in this receiver have a
large stray field and it is necessary to minimize the effect
to as great an extent as possible. The figure-8 type trans-
formers do not have a
large stray field, but when
solenoidal transformers
are used, the inductive
effect between the trans-
formers must be in some
way controlled. One
method of doing this is
shown in Fig. 25.

The three transformers
a,b, and care tilted. The BiG 49
line of force emanating from coil a pass through the winding
of coil b in such a direction that they are approximately
parallel to the wires that constitute the winding of coil b.
If the lines of force from coil a do not cut through any of
the winding of coil b, there will be no coupling effected
between the two coils.

The lines of foree emanating from coil b pass through the
winding of coil c approximately parallel to the wires in that
winding and the lines of force from coil b also pass through
coil c approximately parallel to the wires that constitute
the latter coil. In this manner inductive coupling is
minimized but it is not eliminated entirely. It is worth

42 SRADIO SRECEIVERS

while to have the coils mounted in such a manner that they
can be tilted at any angle, from zero to 90° with the
horizontal. If this is done it is possible to orient the coils
to the point of minimum inductive coupling and this angle
is somewhat critical.

Other Methods of Stabilizing Radio-Frequency Ampli-
fiers.—Besides the potentiometer and neutrodyne methods
of suppressing oscillations, there are other ways of obtain-
ing similar results. In a large number of commercial
receivers, grid resistors are used in the radio-frequency
stages to minimize the tendency to self-oscillation. The
grid resistors, as the name implies, are connected in the
grid circuits of the radio-frequency amplifier tubes. The
most common position is between the grid of the tube and
the stator plates of the tuning condenser. A resistance of
about 1,000 ohms in each of the radio-frequency stages
will in most cases maintain the amplifier in a stable con-
dition.

Another favorite method of stabilizing radio-frequency
amplifiers is to change the overall efficiency of the set by
controlling either the filament or the plate current of the
radio-frequency tubes. In a large number of sets the
rheostat controlling the filament current of the radio-
frequency tubes acts also as a volume control and, inci-
dentally, as an effective means of reducing the energy to
the point where the set remains stable.

Then, there are the so-called “losser’’ methods. A
shorted coil mounted near the tuning coil will absorb
sufficient energy to effect stable operation. Similarly,
by mounting the tuning coils near the variable condenser,
sufficient energy will be dissipated in eddy currents to
obtain the same results.

The proper use of, the shield-grid tube will eliminate any
tendency to self-oscillation and, at the same time, obtain

AND SERVICING 43

extremely high amplification. The manufacturer’s
instructions accompanying this tube should be followed
in all cases.

There are numerous other ways of preventing radio-
frequency amplifiers from oscillating and introducing
distortion into the receiver. Most of these are based on
the introduction of opposing voltages, on proper balancing,
on losses, and on changing the phase relation between the
Individual circuits.

Shielding.—Shielding is a means of confining the mag-
netic fields of coils and conductors to a restricted area.
To be effective, shielding must be designed with proper
relation to the parts to be protected. When correctly
applied, it increases the sensitivity and selectivity of a
receiver because the shields exclude external disturbances
an minimize internal interference.

Electromagnetic shielding, to be effective, must be
complete. The smallest crack or opening is sufficient to
spoil the whole receiver and it is imperative, therefore, that
great pains be taken with the work. Shielding is not
purely a mechanical operation, as it requires technical
design as well, based on the action of the radio-frequency
circuits in the set.

The following facts compiled by the Aluminum Com-
pany of America apply to the art of shielding:

1. Within limits, the effectiveness of shielding increases
with the frequency and with the conductivity and thick-
ness of the metal sheet used.

2. At frequencies in the broadcast range, relatively
thin sheets of aluminum or copper aresatisfactory. Number
20 B. & S. gauge and heavier is used.

3. Aluminum and copper of the same thickness are
equally efficient, for practical purposes, in radio-frequency
shielding.

4—4

44 RADIO. RRC E DYES

4. Complete metal shields of the can or box type, well
grounded are the most effective.

5. Such shields should make good electrical contact at
the joints, and the holes or outlets should be kept as few
as possible.

REFLEX RECEIVER

In a reflex receiver, the amplifier tubes are made to
function as radio-frequency amplifiers and as audio-fre-
quency amplifiers also. The neutrodyne receiver lends
itself best for reflexing, since the radio-frequency amplifier
circuits are so well neutralized that 90 volts can be applied
to the plates of the tubes without danger of unstability of
operation. It is also to be noted that the normal voltage
for an audio-frequency amplifier, using UX-201-A type
tubes, is 90 volts. A radio-frequency receiver that is so
unstable in operation that no more than 45 volts can be
applied to the plates of the radio-frequency amplifier tubes
without making them oscillate is not particularly adapted
for use as a reflex receiver, since the maximum allowable
plate voltage would be limited to 45 volts by the radio-
frequency amplifier tubes. This would mean that the
audio-frequency amplifier tubes would have to be operated
at this potential, which would not be conducive to efficient
reflex amplification, and a separate audio-frequency
amplifier should be used.

The schematic wiring diagram, Fig. 26, shows a receiver
employing three tubes a, b, and c, having two stages of
radio-frequency amplification, a detector, and two stages
of audio-frequency amplification. The constants that
applied in the case of the neutrodyne receiver, apply here
as well, with the addition of two-audio-frequency trans-
formers and several fixed condensers. The transformer
d has a 6 to 1 ratio of turns, while the transformer e has a
2 to 1 ratio of turns.

9¢ “OT

46 . “RADIO SRECEIVERS

The radio-frequency circuits have already been dis-
cussed, hence the audio-frequency circuits which are
reflexed, will now be considered.

The plate terminal of the detector tube c is connected
to one end of the primary winding of the first audio
frequency transformer d. There is a .002-microfarad
radio-frequency by-pass condenser across this winding.
If desirable, regeneration may be effected in the detector
plate circuit by means of a variometer connected in series
with the detector plate lead. This variometer should be
capable of tuning the plate circuit in question to the various
frequencies in the wave band for which the receiver is
designed.

The secondary winding of the first audio-frequency
transformer d is connected in series with the grid return
lead from the tube a. There is a .002-microfarad radio-
frequency by-pass condenser across the secondary winding
of the audio transformer. The function of this condenser
is to offer a low impedance path for the radio-frequency
currents in this part of the circuit. In some types of audio-
frequency transformers the secondary winding of itself has
sufficient capacity to by-pass the radio-frequency currents
without the application of the by-pass condenser.

In cases where a radio-frequency by-pass condenser is
desired in circuits carrying audio-frequency currents, the
value of the by-pass condenser must not be so large as to
by-pass audio-frequency currents as well. The larger the
value of a condenser the lower its impedance to the
passage of alternating currents. The higher the frequency
of the alternating currents the less the impedance of the
condenser to the currents of that frequency. Thus, a con-
denser could have such a value that it would offer a low
impedance to the high-frequency current but would offer a
fairly high impedance to the low-frequency current.

AND, SERVICING A7

The primary winding of the second audio-frequency
transformer e is connected in the plate circuit of the tube a.
The secondary winding of this transformer is connected in
series with the input circuit of the tube 6. Each winding
has a .002-microfarad radio-frequency by-pass condenser
across it.

The tuning condenser in the second stage is connected
from the grid terminal of the tube 6 to the —C terminal, or,
in other words, across the series combination of the secon-
dary winding of the radio-frequency transformer and the
secondary winding of the audio-frequency transformer e.
As far as the tuning is concerned, it is approximately the
same, whether the tuning condenser is connected across
the extremities of the secondary winding of the radio-
frequency transformer or as shown in the figure.

Considering the functioning of this type of circuit it is
found that the radio-frequency input is amplified by the
tube a, then passed on to the tube 6, where it is again
amplified and passed on to the detector tube c. In the
detector tube c the radio-frequency energy is changed
into audio-frequency energy and as such it is applied
back on the grid of the tube a, which amplifies this signal
at audio frequency and passes it on to the second stage of
audio-frequency amplification, which is effected by the
tube 6. The audio-frequency output is taken out of the
plate circuit of the second amplifier tube b. The phones
or loudspeaker f are connected in the plate lead of the
tube b and are shunted by a .002-microfarad radio-fre-
quency by-pass condenser.

SUPERHETERODYNE RECEIVER

Principle of Operation.—The graph in Fig. 27 shows the
fundamental circuits of the superheterodyne receiver.
The name is derived from the fact that the incoming

48 “RADIOS R EE CHLY ERs

signal is heterodyned by a local oscillator. The beat fre-
quency between the incoming signal and the local oscilla-
tor is amplified before application to the detector which
changes the signal into audio frequency.

It will be assumed that the wavelength of the received
signal is 300 meters, which corresponds to 1,000 ke., one
ke. (kilocycle) being the equivalent of 1,000 cycles. A
local radio-frequency oscillator a is coupled to the input
circuit of the high-frequency detector b, so that the signal
frequency and the local oscillator frequency beat together
to give a frequency that is equal to the sum of the two
frequencies in question, and another frequency that is
equal to the difference of the two frequencies in question.
It is the latter that will be considered.

The local oscilla-
tor a is so adjusted
that the beat fre-
| quency will be that
frequency to which
the intermediate-fre-
quency amplifier c is
resonant. In this case the intermediate frequency
amplifier is resonant to energy having a frequency of 30 ke.,
which corresponds to a wavelength of 10,000 meters.
Thus the local oscillator is adjusted to a frequency of either
1,030 ke. or 970 ke.; in either event the difference or the
beat frequency is 30 ke. This gives the reason why it is
possible with a superheterodyne receiver to tune in a
particular station at two different settings of the local
oscillator.

The function of the high-frequency detector 6 is to
rectify the radio-frequency input so that the beat fre-
quency will appear in the output circuit as an alternating
current, having a frequency in this case of 30 ke. This

MTe 27,

AND SERVICING * 49

30-ke. signal is applied to the input circuit of the inter-
mediate-frequency amplifier c. The intermediate-fre-
quency amplifier is the heart of the superheterodyne
receiver, for the fundamental principal involved is that a
signal of the order of 300 meters can be amplified to a
greater degree and with less chance of producing unstability
in the receiver circuits if it is changed to a 10,000-meter
signal and amplified at that wave length.

In the case of a radio-telephone signal from a broadeast-
ing station, the fundamental frequency would be accom-
panied by frequency bands 5,000 cycles wide on either
side of the carrier. In order to produce an undistorted
signal in the audio-frequency amplifier output it is neces-
sary that the intermediate-frequency amplifier be broad
enough to pass all the frequencies in the side bands, which
means that the intermediate-frequency transformers should
be capable of passing a frequency band 10,000 cycles wide,
or from 8,570 meters to 12,000 meters. An intermediate-
frequency transformer that will only pass wavelengths
between 9,000 and 11,000 meters is too sharp, because it
will chop off some of the side bands in the broadcast signal,
which will be manifested by distortion in the audio-fre-
quency output unit. This is the reason for the fact that
some superheterodyne receivers produce a distorted signal.
Their intermediate-frequency circuits are too selective
and they exclude a vital part of the incoming signal.
True sound reproduction is obtained only when all the
side bands (and nothing more) appear in the loudspeaker
output.

The function of the low-frequency detector d is to rectify
the intermediate-frequency signal to produce one of the
5,000 cycle frequency bands for amplification in the audio-
frequency amplifier e. The current in the output circuit
of the detector tube d follows the audio-frequency varia-

50 CRADIO SRECEIVERS

tions in the amplitude of the 30,000-cycle current, which
is due to the modulation frequency at the transmitting
station.

Schematic Diagram.—A schematic wiring diagram of a
superheterodyne receiver is shown in Fig. 28. This set
has one stage of radio-frequency amplification ahead of
the high-frequency detector, three stages of intermediate-
frequency amplification ahead of the low-frequency detec-
tor, and two stages of audio-frequency amplification. In
this receiver there is a jack connection a that allows for
plugging in either an antenna-ground system or a loop.
There is also a change-over switch 6, which can be placed
in either one of two positions. In one position of the
switch the receiver functions as a tuned radio-frequency
receiver with a feed-back in the antenna circuit, employ-
ing four tubes; one radio-frequency amplifier tube, a
detector, and two audio-frequency amplifiers. When the
change-over switch is thrown to the other position, the
receiver functions as a superheterodyne receiver.

The antenna-ground or the loop connections to the
receiver are made through the medium of a plug to the
radio-frequency input jack a. The rotor of the feed-
back coupler c and the primary winding of the first radio-
frequency transformer d are connected in series with the
jack.

The secondary winding of the input transformer d is
tuned to the broadcast wave band by means of a .0005-
microfarad variable condenser. The input transformer d,
in this case, is of the figure-8 type so as to minimize the
possibility of picking up signals directly on this coil. A
receiver of this type is very sensitive and if the solenoidal
type of transformer is used in the input circuit, consider-
able of the signal is picked up directly by the input trans-
former. Thus, in this case, if a loop is used for external

AND SERVICING

AQfiAuly 4

ee ae ae ee Roe |e

oo Poe eae
nid
OFTHE

$°O pad

YAY J

QQVQ0 0

Bo “OI

——
mS

49C j5/

AOHPHISO

x

erat

52 ‘RADIO RECEIVERS

pick-up its directive properties will be impaired, owing to
the pick-up within the set. The figure-8 type of trans-
former minimizes the possibility of direct pick-up within
the receiver at this point in the circuit. The grid return
of the first radio-frequency amplifier tube e is brought to
the contact terminal of the stabilizing potentiometer f.

The plate of the first radio-frequency amplifier tube e is
connected to one end of the stator winding of the feed-
back coupler c, the other end of this winding being con-
nected to the primary winding of the second radio-fre-
quency transformer g. The other end of this latter wind-
ing 1s connected to the positive terminal of the B-battery
supply for the radio-frequency amplifier tubes.

The secondary winding of the radio-frequency trans-
former g is tuned by means of a .0005-microfarad con-
denser. It is at this point in the circuit that energy from
the local oscillator is introduced. This is effected by
means of the coupling coil h, which is in the oscillating
circuit of the separate oscillator and is inductively coupled
to the secondary of transformer g. The grid-condenser
(.00025 microfarad) and the grid-leak (2 megohms) com-
bination 7 is put in series with the grid lead to the
detector tube 7 to cause this latter tube to effect detector
action.

The plate of the detector tube 7 is connected to the
change-over switch b. In the superheterodyne position
the plate lead is connected to one end of the primary wind-
ing of the first intermediate-frequency transformer k.
In the radio-frequency position of the switch the plate
lead is connected to the detector B-battery supply through
the contacts of the jack | and the primary winding of the
first audio-frequency transformer m.

The lower end of the primary of the transformer k is
connected to the positive B-battery supply for the detector

%

AND SERVICING 53

tubes. The secondary terminals of this intermediate-
frequency transformer k are connected to the grid of the
first intermediate-frequency amplifier tube n and the
contact terminal of the stabilizing potentiometer f.

The output of tube n is coupled to the input of the
second intermediate-frequency amplifier tube o through
the medium of the second intermediate-frequency trans-
former. The output of tube o is applied to the grid of the
third intermediate-frequency amplifier tube p.

The output of the third intermediate-frequency ampli-
fier tube p is applied to the grid of the second detector
tube g through the medium of the tuned intermediate-
frequency transformer r. The grid-leak (2 megohms) and
grid-condenser (.00025 microfarad) combination s causes
the tube q to function as a detector. The grid-return lead
of the second detector tube q is brought to the negative
filament terminal. The plate of the second detector tube q¢
is connected to the triple-pole double-throw switch b.
In the superheterodyne position the plate of tube q is
connected to the detector B-battery supply through the
contacts of jack J and the primary winding of the first-
audio-frequency transformer m. ‘The first, or uppermost,
pole of the blade of the change-over switch 6 is connected to
the positive filament terminals of the three intermediate-
frequency amplifier tubes n, 0, and p, the detector tube q,
and the oscillator tube ¢. The terminal that this pole
engages in the superheterodyne position is connected to the
positive A-supply lead through the rheostat wu.

The local oscillator tube ¢ has a tuned Hartley oscillat-
ing circuit. The fact that the .0005-microfarad tuning
condenser is connected from plate to grid insures the fact
that the two elements of the tube in question will always be
180° out of phase, which is the condition for self-oscilla-
tion. The grid excitation for the oscillator tube ¢ is that

54 SRADIO SRECEIVERS

voltage which exists across the grid coil v. The circuit
through the inductive branch of the oscillating circuit
might be traced from the grid of tube ¢ through the erid
excitation coil v, the coupling coil h to the negative fila-
ment terminal. ‘There is a .1-microfarad radio-frequency
by-pass condenser wu from the negative filament lead to the
positive radio-frequency B-battery lead. The circuit is
traced from the positive radio-frequency B battery through
the plate coil x to the plate terminal of oscillator tube t.
In effect, the coils v and x are joined together at their
inner ends by virtue of the radio-frequency by-pass con-
denser w, and this point is at ground potential as far as
radio frequency is concerned. The condenser w also func-
tions as a blocking condenser in the oscillator circuit, allow-
ing the plate potential to be supplied at the mid-point
between plate and grid, the condenser w blocking the d.-c.
potential from being applied to the grid of the oscillator
tube 1.

When the change-over switch b is thrown to the super-
heterodyne position, all the tubes shown in the figure are
in operation. The condenser in the radio-frequency and
first-detector circuits tune their respective circuits to the
incoming radio-frequency signal and the oscillator con-
denser is set to give the desired 30-ke. beat frequency, the
intermediate-frequency amplifiers functioning at this
frequency.

For local reception, the change-over switch b is thrown
to the radio-frequency position, thereby cutting off the
filament-current supply to the tubes n, 0, D4 OG, ae
The plate of tube 7 is cut off from the primary winding of
the first intermediate-frequency transformer k and is con-
nected to the detector B-battery supply through the con-
tacts of the jack J and the primary winding of the first
audio-frequency transformer m, and the plate of tube q 1s

AND SERVICING 55

disconnected from its output circuit. Thus, only the
tubes e, 7, y, and z are left in operation, functioning as one
stage of radio-frequency amplification, detector, and two
stages of audio-frequency amplification, respectively.
This will be found to be adequate for the satisfactory
reception of local signals.

List and Constants of Parts.—The parts used in the
construction of the set shown in Fig. 28 may be purchased;
some of them, however, may be readily constructed by the
experimenter. The rotor and stator of the coupler c¢ are
each wound with 30 turns of No. 24 d.s.c. wire, the stator
on a 3-inch form and the rotor on a form to fit inside the
3-inch form.

The transformer d is of the twin-8 or double-D type.
The primary has 15 turns No. 24 d.s.c., and the secondary
has 50 turns No. 24 d.s.c. wire.

The transformer g has three windings. The primary
winding consists of 18 turns No. 24 d.s.c. wire wound on a
23-inch form; the secondary, 50 turns; and the oscillator
coupling coil h, 2 turns, all wound on the same form.

The intermediate-frequency transformers should have a
working range between 8,000 and 12,000 meters.

The oscillator coils v and x are wound on a 23-inch form
with No. 24 d.s.c. wire, the grid coil v having 27 turns, and
the plate coil z, 28 turns.

The three variable condensers have each a capacity of
.0005 microfarad. The condenser w has a capacity of .1
microfarad. The two detector grid condensers are .00025
microfarad each. The grid leaks are 2 to 3 megohms each.
The potentiometer f is of the 400-ohm type. The rheo-
stat u has a resistance of 6 ohms.

56 CRADIO SRECEIVERS

SHORT-WAVE RECEIVERS

Peculiarities of Short-Wave Reception.—The four most
popular short-wave bands at the present time are the
20-, 40-, 80-, and 100-meter bands. The logical thing to
do is to use a separate coil for each of the three first-men-
tioned wave bands in the list of four here given. It will
be found that the coil for the 80-meter band will also suffice
for the 100-meter band.

In short-wave reception it is not desirable to use a tun-
ing condenser having a maximum value of capacity greater
than .00025 microfarad. Probably a tuning condenser
saving a maximum capacity value below .0002 microfarad
is still more desirable because the tuning in the short-wave
band is far different from the tuning in the broadcast-wave
band. This can be explained by the following facts that
were received from actual practice.

A .00025-microfarad variable condenser is shunted
across a coil of such an inductance value that the combina-
tion tunes to 20 meters with the condenser dial set at 10.
This combination tunes the circuit to 21 meters with
the condenser dial set at 11. The frequency of a 20-meter
wave is 300,000,000 +20= 15,000,000 cycles per second
which is 15,000 ke. The frequency of a 21-meter wave
is found to be 14,800,000 cycles, or 14,300 ke. Thus, by
rotating the tuning condenser through | division of the dial,
a 700-ke. frequency band has been covered.

Since the signals from a radio broadcasting station cover
a frequency band about 10,000 cycles, or 10 ke. wide, the
20-meter broadcaster would be passed over with a rota-
tion of the tuning control of ~;th of a dial division.

On the other hand, consider the tuning around 500
meters. With the same .00025-microfarad tuning con-
denser and a coil of proper inductance the circuit is tuned

AND SERVICING 57

to a wavelength of 500 meters with the condenser dial at
80. This is approximately the dial setting for this wave-
length in the standard type of broadcast receiver. Now,
if the tuning condenser is rotated through 2.6 divisions
to 82.6, the wavelength will be increased to 508 meters.
The frequency at 500 meters is 600 ke. and the frequency at
508 meters is 590 ke., the difference being 10 ke., or the
frequency band of a broadcasting station. Thus, it can
be seen that the tuning condenser dial must be rotated
through 2.6 divisions to pass through the signals from a
500-meter broadcasting staton. <A consideration of the
foregoing analysis will afford a good idea of the reason
why broadcast signals appear to afford much sharper tun-
ing on the shorter wavelengths. In the realm of the
extremely short waves the tuning condenser must not be
too large or tuning will be very difficult.

The consensus of opinion, among the uninitiated, is
that there are only radio telegraph signals on the short
waves, but this idea is fallacious. It is true that most of
the activity on the short waves is radio telegraph communi-
cation, but the fact remains that there are also some broad-
casting stations operating on the short waves. For
instance, the Pittsburgh station KDKA broadcasts pro-
grams on 26.38, 42.95, and 62.5 meters and WGY at
Schenectady broadcasts on 35 meters. It is quite cus-
tomary for both of the stations just mentioned to broad-
cast the same program on a short wave that is being broad-
casted on their normal broadcasting wavelength. There is
less static on the short waves and often a broadcast pro-
gram can be heard on the short wavelength of a station that
can hardly be heard or not picked up at all on its normal
broadcasting wavelength.

Short-Wave Receiver With Interchangeable Coils.—In
Fig. 29 (a) is shown a schematic wiring of a short-wave

58 CRADIO SRECEIVERS

receiver that can be used to receive signals between the

limits of 17 and 130 meters.
is shown in view (0).

The arrangement of parts

The same reference letters are used

in both views to indicate corresponding parts.

Hel 4
Cena Re!
ULLAL EEE izle

l
| +B Amp.

+B Det:

=B+A
-A

SS
—

D

The 10-turn coil a is the coupling coil in the aperiodic,

or untuned, antenna circuit, through the medium of which
the radio-frequency energy picked up by the antenna
system is induced into the coil b, which forms part of the

tuned circuit.

The coupling between the coils a and 6 is

variable.

AND) {SERV ILCING 59

The ground lead is connected through to the low, or
filament, side of the secondary coil b so as to reduce body
capacity. However, it will be found that there will be less
interference from local sources such as street lighting cir-
cuits, subways, street cars, and elevated systems, and
alternating-current induction from house lighting circuits
if the ground is not connected through to the negative
filament lead.

The feed-back coil c is closely coupled to the filament
end of the secondary coil b. Tuning is effected by means
of the condenser d. Regeneration is controlled by means
of the condenser e. This method of effecting regeneration
is quite similar to the capacity-controlled regenerative
circuit and it is used because of the fact that the regenera-
tion control varies only slightly with frequency, so that a
single setting of the control in question may be used for a
wide band of wavelengths. This feature is conducive to
a small amount of variation in the tuning due to the
adjustment of the feed-back circuit. It follows, then, that,
since there are only the two controls d and e, if the latter is
such as to require a very small amount of adjustment, the
receiver becomes practically single control, which is quite
a desirable feature for a short-wave receiver.

The grid leak f should be made as large as possible and
the grid condenser g as small as possible. A satisfactory
combination is 5 megohms and .0001 microfarad, respec-
tively. The positive potential for the plate of the detector
tube h is supplied through the primary winding of the
audio-frequency transformer 7, the radio-frequency choke 7,
and the coil c. The function of the radio-frequency
choke 7 is to keep all radio frequency out of the audio-
frequency circuit, where it might cause howling, and also
to prevent the distributed capacity of the phones or the
primary winding of the audio-frequency transformer from

4—)

60 SR ADIO SRECEIVERS

shorting the radio-frequency currents around the oscila-
tion-control condenser e.

The tube & with its auxiliary apparatus provides a stage
of audio-frequency amplification. The jack | constitutes
a convenient means of connecting the phones or loud
speaker in the output circuit. The filament current for
both tubes is controlled by the rheostat m.

The constants that remain unchanged for all the wave-
lengths between 17 and 130 meters are given herewith.

a—10 turns No. 24 d.c.c. wire, 3-inch diameter, sole-
noid winding.

d—.00014-microfarad variable condenser.

逗.00025-microfarad variable condenser.

f—5-megohm grid leak.

g—.0001-microfarad fixed condenser.

h—UX-200-A detector tube and socket.

7—Low-ratio audio-frequency transformer.

j—200 turns No. 36 d.s.c. wire, 1-inch diameter, sole-
noid winding.

k—UX-201-A amplifier tube and socket.

/—Output jack.

m—10-ohm rheostat.

From a consideration of the foregoing list it will be seen
that the only elements that change for covering the dif-
ferent wave bands within 17 to 130 meters are the secon-
dary inductance 6 and the feed-back coil c. Their
constants are as follows:
20-Meter wave Band
_ 6—8 turns No. 18 bare copper wire, 3-inch diameter,

spaced the diameter of the wire.

c—2 turns No. 24 d.c.c. wire, 3-inch diameter, no spacing.
40-Meter Wave Band

6b—8 turns No. 18 bare copper wire, 3-inch diameter,
spaced the diameter of the wire.

AN Deny ERY 1.1 NiG 61

c—4 turns No. 24 d.c.c., 3-inch diameter, no spacing.
80-Meter Wave Band

b—19 turns No. 18 bare copper wire, 3-inch diameter,
spaced the diameter of the wire.

c—6 turns No. 24 d.c.c., 3-inch diameter, no spacing.

In Fig. 30 are shown the wavelength callibration curves
for the 20-, 40-, and 80-meter bands, the respective coils
designated for those particular wave bands being used.

20,000
18,000
16,000
ey 4
2/4000 &
5S) y
at 4
% &
8/2000 =
x &
8 /0,000 &
: Ww
Q Ny
& 8000 G
xX
S
, g 6,000 8
4000
2000 ana

LOE ZO NIO MFO SOM COM COMCOMmIOMIOO:
Condenser Dial Setting

Fig. 30

These curves were made with a straight-line frequency
condenser at d, Fig. 29; thus any straight-line-frequency
condenser having the same maximum and minimum
capacity limits will give practically the same shape of
curve. If a condenser having a straight-line capacity
curve is used, having the same maximum and minimum
limits as the condenser used for obtaining the curves shown
in Fig. 30, the range will be the same but the shape of the
curves will be different.

With the 20-meter coil in operation it is possible to tune
from 8,900 ke. (33.7 meters) to 18,000 ke. (16.7 meters).

62 SR ADIO CRECEIVERS

The majority of transmitters in operation on the 20-
meter band will be tuned in between the two vertical lines
on the curve in question, namely, between 18 meters
(16,700) ke.) and 22 meters (13,600 ke.).

With the 40-meter coil in circuit, it is possible to tune
from 4,300 ke. (69.8 meters) to 9,500 ke. (31.6 meters).
The active part of this curve is that within the so-called
40-meter band, which is that part of the curve included
between the two vertical lines, namely, between 36 meters
(8,330 ke.) and 42 meters (7,150 ke.).

With the 80-meter coil in operation it is possible to tune
from 2,200 ke. (1386 meters) to 5,400 ke. (55.5 meters).
The active part of this
curve is that portion which
is designated as the 80-
meter band and_ which
includes the wavelengths
between 77 meters (3,900
ke.) and 83 meters (3,620
ke:).

Short- Wave Throttle
Tuner, 17 to 90 Meters.
In Fig. 31 is the schematic
wiring diagram of a receiver for covering the 20-, 40-, and
80-meter wave bands. As discussed in the case of the
preceding receiver, it is quite essential that a method of
regeneration be effected that has little effect on the tuning
of the receiver input circuit, and it is also desirable that
this feed-back be controlled in such a manner that it will
not need a great deal of adjustment over a wide variation
in tuning. In the preceding circuit arrangement a special
method of regeneration control was effected having the
desired characteristics. In this circuit the feature is the
throttle method of regeneration control.

SF
sf

HiGy.o1

WEAN DY SERV LOIN'G 63

There are only two tuning ranges with this receiver and
one set of constants. The two ranges are effected through
the medium of a single-pole double-throw switch. The
following is a list of the receiver constants.

a—10 turns No. 18 d.c.c. wire, 34-inch diameter, no
spacing, tapped at fifth turn.

b—4 turns No. 18 d.c.c. wire, 14-inch diameter, no spac-
ing. .

c—.00001-microfarad variable condenser.

d—.00018-microfarad variable condenser.

e—.0001-microfarad fixed grid condenser.
f—.00025-microfarad variable throttle condenser.
g—5-megohm grid leak.

h—10-ohm rheostat.

i—Single-pole, double-throw switch.

j—UX-200-A tube and socket.

k—200-turns No. 36 d.s.c., 1-inch diameter, solenoidal
winding.

The only thing about this receiver that needs particular
explanation is the construction of the two coils a and b.
They are both of the low-loss type. In constructing these
coils, scribe a circle 34 inches in diameter on the surface of
a piece of wood, the latter being preferably about > inch
to 2 inch thick. Locate 11 equidistant points around the
circumference of this circle, as shown / to 1/1, Fig. 32, and
fasten a 2 by 53-inch stud at each one of these points.
These studs can be made by hammering through a 23-
inch nail at each of the eleven points.

In winding the coils begin at a and pass the No. 18 d.c.e.
wire, on the outside of the first stud, on the inside of studs
2 and 3, on the outside of stud 4, etc., always passing the
winding outside of one stud and inside of the succeeding two
studs. The coil a, Fig. 31, is composed of 10 turns, with a
tap at the mid-point. The coil 6 is composed of 4 turns.

64. RIAD T Om Rie CE diy Eeeas

The antenna is coupled to the receiver input circuit
through the .00001-microfarad condenser c, Fig. 31.
This little coupling condenser may
be variable or it may be fixed.
Possibly some advantage may be
derived by having a variable con-
denser.

The coil a is tuned by means of
the .00018-microfarad variable con-
denser d. It is possible to tune
through two ranges of wavelengths
by means of the double-throw
switch 7 and the tapped coil a. With the switch 7 con-
nected to the fifth-turn tap on the 10-turn coil a, it is
possible to tune from 18 to 51 meters with the tuning con-

cept
ead
ewes
ou
eae

H1G.32

RG
EIS,
A Tekonl |

Wavelength in Meters

nian
eer

30 40
Cond. bial is ttin

Dike, 6%}

denser d. With the switch 7 engaging the tenth turn on
coil a it is possible to tune from 35 to 90 meters.

AND SERVICING 65

The coil 6 has a fixed inductive relation to the coil a,
the former being located 2 inches from the grid end of the
latter. This amount of coupling is sufficient to effect the
minimum degree of regeneration, and, from this point up
to the maximum, is controlled by means of the throttle
condenser f.

The radio-frequency choke k prevents the distributed
capacity of the phones or primary winding of the first
audio-frequency transformer, when used, from shorting
the radio-frequency energy around the throttle condenser f.

A chart showing the two wavelength curves for the
two settings of the wave-change switch 7 is shown in
Fig. 33. The lower curve covers the 20- and 40-meter
band and the upper curve covers the 40- and 80-meter
bands.

SINGLE-SIDE-BAND RECEIVER

In view of the outstanding advantages of the single-side
band eliminated-carrier system for long-distance radio-
telephone communication, the American Telephone and
Telegraph Co. have established a commercial radio-
telephone system wherein this system is used at the
transmitting end, and a receiver designed for the reception
of signals of this character is used at the receiving end. It
seems logical to assume that this type of transmission and
reception will be used more and more during the coming
years and it is probable that broadcast programs will be
sent out at some future time by single-side-band trans-
mitters.

In the discussion concerning the transmitter, the circuit
constants given were those of the transmitter that was
used in the successful two-way transatlantic radio tele-
phone tests, the transmitter being later used for commercial
transatlantic telephone communication. In order to link
up the receiver discussion with that of the transmitter,

66 (RADIO CRECEIVERS

the receiver-circuit constants given herein will be for the
reception of signals emanating from the transmitter in
question, it being the most powerful single-side-band
eliminated-carrier transmitter in operation at the present
time, having a radio-frequency output of 200 kw.

A schematic wiring diagram of a simple receiver of this
type that was used to pick up the signals during the trans-
atlantic tests is shown in Fig. 34. The circuit constants
are as follows: pA

a—500-turn honeycomb coil.

b—750-turn honeycomb coil.
c—300-turn honeycomb coil.
d—25-turn honeycomb coil.

AND SERVICING 67

e—300-turn honeycomb coil.

f—500-turn honeycomb coil.

g—.0005-microfarad variable condenser.

h—.0005-microfarad variable condenser.

7—.0005-microfarad variable condenser.
g—.00025-microfarad grid condenser.

k—.1-microfarad fixed condenser.

[—.002-microfarad fixed condenser.

m—12-ohm rheostat.

n—12-ohm rheostat.

o—3-megohm grid leak.

p—UX-200-A detector tube and socket.

g—UX-201-A amplifier tube and socket.

Considering the schematic diagram in Fig. 34, the
antenna is connected to one side of the tuned circuit con-
posed of the 500-turn coil a and the .0005-microfarad
tuning condenser g. The other side of this parallel com-
bination is connected to ground. It is by means of this
circuit that the antenna-ground system is tuned to reson-
ance with the incoming side band.

The energy in the antenna system is induced into the
detector-tube input circuit by means of the inductive
coupling between the antenna coil a and the grid coil b.
The latter is a 750-turn coil and is tuned by means of the
.0005-microfarad tuning condenser h. Detector action is
effected by means of the combination of leak 0 and con-
denser j7. Regeneration is obtained with the feed-back
coilc. The .002-microfarad by-pass condenser / shorts the
radio-frequency energy in the detector plate circuit around
the phones or the primary winding of the first-audio
frequency transforrher, if such is used. The receiver, as
described so far, will function quite well in the reception of
single-side-band signals if the degree of regeneration is
increased to the point of oscillation.

68 SR ADIO CRECEIVERS

The single side band that is intercepted by the receiv-
ing antenna has a frequency band of from 55,800 cycles to
58,500 cycles. In meters, this is 5,370 meters to 5,120
meters. It is to be remembered that the function of the
transmitter in this case is to transmit good quality speech
but not necessarily music or frequencies higher than the
speech frequencies. Good quality speech can be trans-
mitted with a frequency band of from 300 to 3,000 cycles.

The received signal only occupies a 2,700-cycle frequency
band, whereas, in normal transmission on this wavelength
where the carrier and both the upper and lower side bands
are transmitted, the frequency band would be twice as
wide.

If the constants of the receiving circuit are adjusted so
that the detector sets up oscillations having a frequency of
55,500 cycles, this frequency will remodulate, or beat, with
the received side band of 55,800 to 58,500 cycles, and a dif-
ference-frequency band of 300 to 3,000 cycles, or the voice-
frequency band will result. Thus, there is nothing compli-
cated necessary in the reception of signals of this type, the
old regenerative detector circuit being quite capable of
giving satisfactory results.

It is possible to carry the development of this receiver
a step farther and use a separate oscillator at the receiving
station to beat with the incoming signal to produce the
voice-frequency band. The application of the separate
oscillator is also shown in the figure, but is not considered
entirely necessary for satisfactory results.

The oscillatory circuit for the separate oscillator is of
the tuned Hartley type, the grid of the oscillator tube q
being connected to one end of the oscillatory-circuit
inductance edf and the plate to the other end. The fila-
ment-ground is connected to an intermediate point on the
inductance in question. Tuning is effected by means of a

ANE Dag) Ee RV Eb CTEN G 69

.0005-microfarad variable condenser k shunted across the
entire oscillatory-circuit inductance edf. The 500-turn
coil f is the plate coil and the 300-turn coil e is the grid
excitation coil. A 25-turn coil d is used for coupling the
oscillator to the receiver input circuit.

Although the simple arrangement described in the
preceding paragraphs is satisfactory for amateur work, it is
not quite elaborate enough for a commercial installation,
and a receiver of the superheterodyne type is used. The
general outline of a commercial type of single-side-band
receiver 1s shown in Fig. 35.

The radio waves are intercepted by the loop a, and this
energy is applied to the input circuit of a high-frequency

Hire@ars)

detector b. Energy from a separate 90,000-cycle oscil-
lator cis applied to the same input circuit. The difference-
frequency band of 31,500 to 34,200 cycles that appears in
the output circuit of the detector tube is passed through the
band-pass filter d to the intermediate-frquency amplifier e,
As before, there is only one side band, 31,500 to 34,200
cycles, the frequency of the 90,000-cycle supplied carrier,
and the upper side band due to the beating of the 90,000-
cycle frequency with the signal input having been elimi-
nated by the band-pass filter.

_ The single-side-band output of the intermediate-fre-
quency amplifier is passed on to a low-frequency detector f.
Another oscillator g at this point in the circuit supplies a
34,500-cycle carrier, which beats with the 31,500- to
34,200-cycle side band, the result in the detector output

70 FRADIO SRECEIVERS

circuit being the voice-frequency band, 300 to 3,000 cycles.

From the second detector, the audio-frequency energy

passes to the audio-frequency amplifier h.
AUDIO-FREQUENCY AMPLIFIERS

TYPES OF AUDIO-FREQUENCY AMPLIFIERS

After taking up the discussion of the different types of.

detector and radio-frequency amplifier circuits, consider-
ation will now be given to the various methods of obtain-
ing efficient audio-frequency amplification.

There are three fundamental types of audio-frequency
amplifiers; namely, transformer coupled, resistance
coupled, and impedance, or choke-coil, coupled. The
writer has personally constructed each of the audio-
frequency amplifiers that are discussed in the following
pages, and a list of the circuit constants and the apparatus
used, in each case, is provided.

TRANSFORMER-COUPLED AUDIO-FREQUENCY AMPLIFIER

The first audio-frequency amplifier circuit to be con-
sidered is that of a transformer-coupled unit. When the
proper parts are used in a properly designed circuit, this
type of audio-frequency amplifier is paramount. The
schematic wiring diagram for this amplifier is shown in
Fig. 36 and the following is the list of material that was
used in its construction, as well as the circuit constants.

a—High-ratio audio-frequency transformer (6 to 1).

6—Low-ratio audio-frequency transformer (2 to 1).

c—Output transformer (1 to 1).

d—.002-microfarad radio-frequency by-pass condenser.

e—live 1-microfarad audio-frequency by-pass con-
densers.

f—6-ohm rheostat.

g—Two 500,000-ohm potentiometers.

Ce a

AND IS RIVE CEN'G 71

h—U X-200-A detector tube and socket.

1—UX-201-A amplifier tube and socket.

j—UX-171-A power amplifier tube and socket.

The output circuit of the detector tube h is shown in the
drawing to facilitate the explanation of the method of
coupling the output of the detector tube to the amplifier
input. A variometer may be connected in the detector
output circuit to tune the plate circuit to the frequency of
the incoming radio-frequency signal and thus effect
regeneration. In this case it is necessary to have a radio-
frequency by-pass condenser d from the high side of the
primary winding of the first audio-frequency transformer a

Fic. 36

to the negative filament terminal, in order to by-pass the
radio frequency in this part of the circuit around the
primary winding of the transformer a and the B battery.

The condenser ¢ in the plate circuit of the detector tube h
functions to by-pass audio-frequency currents around the
detector B battery. There is a 500,000-ohm potentio-
meter g whose extremities are connected across the term1-
nals of the secondary winding of the first audio-frequency
transformer a. The contact terminal of this potentiometer
is connected to the grid terminal of the first audio-frequency
amplifier tube 7. This potentiometer functions not only
as a volume control (because the value of the signal voltage

72 SRUA Dl Oe eRVE GLE tvanaRes

applied to the grid of tube 7 can be varied by moving
the contact terminal from the low voltage end of the resis-
tance element to the high-voltage end), but it also tends
to flatten out the transformer characteristic and make the
transformer amplify all frequencies alike.

There is an audio-frequency by-pass condenser e from
the low side of the secondary winding of transformer a to
the negative filament terminal, or, in other words, across
the C battery for this tube. There is another audio-
frequency by-pass condenser e from the low side of the
primary winding of the second audio-frequency trans-
former b to the negative filament terminal of the tube 7, or,
in other words, across the B-battery supply for the tube 7.

The connections of tube j, or the second amplifier tube,
are practically the same as for the tube 7. The output of
the amplifier tube 7 is obtained through the medium of the
output transformer c. This transformer has a one-to-one
ratio and functions to pass the amplified audio-frequency
signal on to the loudspeaker unit and at the same time
keep the high-potential direct current in the plate circuit
of the last tube from passing through the windings of the
loudspeaker field coils. This latter effect is undesirable
from the standpoint that the current, if in the wrong direc-
tion, will tend to demagnetize the permanent magnetism
of the speaker field-coil core, and will tend to bias the
diaphragm of the speaker unit, thus effecting distortion.

If the tubes mentioned in the material list are used,
the A-battery potential is 6 volts and the B- and C-bat-
tery potentials are as indicated in Fig. 36. It is neces-
sary to use some sort of power tube in the second audio-
frequency amplifier stage to prevent distortion due to
overloading of the tube in question; therefore, in this case a
UX-171-A tube with 180 volts on the plate and —40.5 volt
bias has been used.

AND SERVICING 73

IMPEDANCE-COUPLED AUDIO-FREQUENCY AMPLIFIER

Before the development of audio-frequency transformers
for use in radio-broadeast receivers reached its present
stage of perfection, the thought took root in some sections
that transformer-coupled audio-frequency amplification
was not conducive to good quality, although it was
admittedly the best as far as voltage amplification was
concerned. However, when the general broadcast public
began to give up the quest for DX reception and expressed
a desire for good quality reception, the transformer manu-
facturers put their engineers on the problem of producing a
better audio-frequency transformer that would allow for
undistorted amplification. In the meantime the imped-
ance-coupled and the resistance-coupled audio-frequency
amplifiers sprang up as an answer to the problem of obtain-
ing distortionless audio-frequency amplification. In both
eases it was admitted that they were not as efficient as
transformer-coupled amplifiers from a standpoint of
voltage amplification, hence more stages were required,
but it was claimed that they were capable of producing a
better quality output than the transformer-coupled
amplifiers that employed the audio-frequency transformers
on the market at that time. This was probably true.
The impedance-coupled and resistance-coupled amplifiers
probably did procure a better-quality output signal than
could be obtained from the coupling transformers that
were available for radio broadcast fans in the early days
of broadcasting.

A schematic wiring diagram of a choke-coil coupled
audio-frequency amplifier is shown in Fig. 37. The fol-
lowing is the list of the parts that were used by the author
in the construction of a three-stage impedance-coupled
audio-frequency amplifier.

74 fRADIOUCR Bic ETERS

a—Four 200-henry choke coils. _

b—.002-microfarad radio-frequency by-pass condenser.

c—Three 1-microfarad audio-frequency coupling con-
densers.

d—Two 1-microfarad grid by-pass condensers.

e—Three 1-microfarad plate by-pass condensers.

f—Ouput transformer (1 to 1).

g—6-ohm rheostat.

h—Two 500,000-ohm potentiometers.

i—UX-200-A detector tube and socket.

4—Two UX-201-A amplifier tubes and sockets.

k—UX-171 power-amplifier tube and socket.

_ The amplifier shown schematically in Fig. 37 is capable
of producing about the same output volume as that shown

eh

Fig. 37

in Fig. 36, thus one more tube is required in an impedance-
coupled amplifier than in a two-stage transformer-
coupled audio-frequency amplifier to obtain the same
amount of volume.

If there is a feed-back for radio frequency in the plate
circuit of the detector tube 7, Fig. 37, the condenser b is
required to by-pass radio frequency in this circuit around
the plate choke a and the detector B battery.

AND SERVICING 75

The maximum audio-frequency signal input is available
across the first 200-henry choke coil a, hence across the
points 7 and 2. There is another circuit between the
points 7 and 2 besides that afforded by the choke coil a;
namely, that circuit extending from point 7 through the
audio-frequency coupling condenser c, the grid resistance h,
the audio-frequency by-pass condenser d to the negative
filament lead, the audio-frequency by-pass condenser e
to point 2. If the combined capacity reactance of the
coupling condenser c and the by-pass condenser e is small
- relative to the grid resistance h, the signal voltage across
the coil a is practically apparent in its entirety across the
grid resistance h, owing to the fact that the voltage drop
across the condensers named is negligible and the signal
voltage across the coil a is impressed across the series
combination mentioned.

Thus it can be seen that the audio-frequency by-pass
condensers should offer a low resistance to the passage of
audio-frequency currents. The value of the capacity
reactance of a condenser is equal to $zfC, where 7z is the
constant 3.1416, f is the frequency in cycles per second, and
C is the capacity in farads. Since, for a given value of
capacity, the lower the frequency the higher its reactance,
in choosing the proper value of capacity to use as an audio-
frequency by-pass condenser, it is well to consider the
lowest frequency that it is called upon to by-pass, which is
in the neighborhood of 50 cycles.

With the frequency term fixed at 50 cycles per second
in the formula for capacity reactance, the capacity term
can be varied and the change in reactance noted. It will
be found that a condenser having a capacity of .01 micro-
farad offers a reactance of over 300,000 ohms, whereas, a
1-microfarad condenser brings this reactance value down
to 3,000 ohms, approximately. The resistance h, which

4—6 ;

76 FRADIO “RECEIVERS

is in series with the coupling condenser c as far as the signal
voltage is concerned, has a maximum value of 500,000
ohms, so it can be seen that the reactance of condenser c
is negligible if it has a value of 1 microfarad when signal
frequencies of the order of 50 cycles are being put through
the amplifier circuit.

The upper limit of the frequency band in the audio-
frequency signal that is received from a broadcasting
station is of the order of 5,000 cycles and a .01-microfarad
condenser offers a reactance of only 3,000 ohms at 5,000
cycles and a 1-microfarad condenser, 32 ohms at the same ~
frequency. ‘Thus, while a .01-microfarad coupling con-
denser is adequate for the higher frequencies in the
received audio-frequency band, it is too small to be con-
ducive to the satisfactory amplification of the lower fre-
quencies in the received band, and if these low frequencies
are omitted the signal quality is impaired.

The variable resistance h functions as a grid leak. It
prevents the coupling condenser c from charging up to a
sufficient degree to block the tube 7; in other words, it
allows electrons to leak off as fast as they arrive. Having
this element in the circuit, a variable, allows for getting
the most volume out of the amplifier.

An output transformer f is used to supply the amplified
audio-frequency signal to the loudspeaker field coils, at
the same time keeping the high-potential direct current
from passing through these windings.

If the tubes used in this amplifier are as listed in the
material list, the A-battery potential is 6 volts and the B
and C battery potentials are as indicated in the figure.

It is to be noted that the values of the bias-battery
potentials for the different plate potentials are less in this
case than in the case of the transformer-coupled amplifier,
because there is a greater drop in potential through the

AND SERVICING hy

choke-coil winding in the plate circuit of an amplifier tube
than there is through the primary winding of an audio-
frequency transformer. This means that in the case of
choke-coil coupling the effective plate potential will be
lower, hence a lower value of biasing potential is required
than in the case of transformer coupling. Bearing this
fact in mind it will be noted that the bias-battery values
in the case of a resistance-coupled amplifier are still lower
for the same values of plate potential, than in the case
of the transformer-coupled amplifier and the choke-coil
coupled unit.

TRANSFORMER-RESISTANCE COUPLED AUDIO-FRE QUENCY AMPLIFIER

A resistance-coupled amplifier, like an impedance-
coupled amplifier, is conducive to obtaining good-quality
output signals. One stage of transformer-coupled and
two stages of resistance-coupled amplification are quite a
popular circuit arrangement. Such a circuit arrangement
is shown in Fig. 38 and the following is the list of the mate-
rial used, as well as the circuit constants.

a—First-stage audio-frequency transformer (6 to 1).

b—Output transformer (1 to 1).

c—.002-microfarad radio-frequency by-pass condenser.

d—1-microfarad audio-frequency by-pass condenser.

e—Two 1l-microfarad coupling condensers.

f—Two 1-microfarad grid by-pass condensers.

g—Two 1-microfarad plate by-pass condensers.

h—6-ohm rheostat.

i—Two 100,000-ohm variable resistances.

4—One 500,000-ohm variable resistances.

k—UX-200-A detector tube and socket.

I—Two UX-201-A amplifier tubes and sockets.

m—UX-171 power-amplifier tube and socket.

n—200-henry choke coil.

78 Fo SROACD LOR Te Orn Iver eRes

As shown in the schematic wiring diagram, the trans-
former a is connected in the circuit in the same manner as
previously described. In the resistance-coupled circuits
the output resistances 7 perform the same function as the
impedance coils in the impedance-coupled amplifier
described in the preceding text. It is convenient to have
the resistances in the resistance-coupled amplifier variable,
as this allows for getting the optimum degree of efficiency
possible, although .a properly designed fixed resistance
gives excellent results.

Feed-bLack

LGN: [\ {\
CNIS | jae i # B ome
cate

7 Bi. 50
CAEAIAS | ihr <atnh wieteathe the C

+B4S
et ee eee +A-B
(ies eee | —A

Fia. 38

If the tubes used in this amplifier are as listed, the
A-battery potential is 6 volts and the B- and C-battery
potentials are as indicated in Fig. 38. A relatively small
amount of bias is required for the grids of the amplifier
tubes whose plates are supplied with potential through
coupling resistances, owing to the high potential drop
through the resistances in question. This drop is so great

AND SERVICING 79

that, even though the plate-supply sources are 150 and
180 volts, the actual effective potential at the plate termi-
nals is considerably less than this.

!

POWER AMPLIFIERS AND POWER PLATE SUPPLY

ADVANTAGES OF POWER AMPLIFIERS

The design of audio-frequency transformers for use in
broadeast receiving sets has reached such a high degree of
perfection that, if the right ones are used in the proper
circuits, excellent reproduction of broadcast programs may
be obtained.

If a signal of sufficient volume to operate a loudspeaker
so that it can be heard all over a six-room home is taken
out of the plate circuit of a tube of the UX-201-A type,
there is bound to be distortion and it does not necessitate
a very critical ear to notice it. This distortion is due to
the fact that the output tube is being overloaded.

The UX-201-A type tube was designed for use as a
radio-frequency amplifier, detector, and audio-frequency
amplifier (up to a certain degree), but real power cannot be
obtained from anything less than a power tube. It is
true that tubes of the UX-171 type were designed to
answer the requirements of a power tube in a receiving set,
but if high-quality amplification is desired with plenty of
volume, it is necessary to use a tube of the UX-210 type,
which is a 7.5-watt power tube. This tube with 500 volts
on its plate and a bias of 40.5 volts is capable of producing
a high degree of distortionless output energy.

Tubes of the UX-201-A type are capable of producing a
distortionless signal of relatively low value, above which
value the signal becomes distorted, owing to the overload-
ing of the tube. By overloading is meant that the ampli-
tude of the signal voltage applied to the grid of the tube is

80 SReADIO SRECEIVERS

sufficient to produce a value of plate current that is off
the straight part of the plate-current grid-voltage character-
istic curve of that type of tube. As long as the voltage
swing of the grid is low enough to keep the values of
plate current along the straight part of the characteristic
curve, a certain amount of positive potential on the
grid of the tube will produce the same amount of plate-
current change that the same amount of negative grid
potential will cause. However, when the swing of the
grid potential is boosted to an abnormal value by trying
to obtain too great a signal output from too small a
tube, the plate-current values are carried off the straight
part of the characteristic curve, and grid swings in the
positive direction will cause different changes in the plate
current than are caused by equal swings in the opposite
direction. This is manifested by distortion in the signal
output.

Thus, a tube of the UX-201-A type is satisfactory for a
certain amount of volume and beyond this value of volume
it will distort the signal, no matter how good the audio-
frequency transformers are and no matter how good the
loudspeaker is. The thing to do is to use a power tube
and put a high potential on its plate and an adequate
bias on the grid.

It is not economical to obtain the 500 volts for the plate
supply to a UX-210 tube from a dry-battery source. The
logical manner in which to obtain this high d.-c. potential
is to step a 110-volt a.-c. supply up to approximately 550
volts by means of a step-up transformer. This high-
voltage alternating current can then be rectified and passed
through a filter circuit, which will smooth it out to approxi-
mate direct current to the extent that it can be used as a
source of constant potential for the plate of the tube in
question.

AND SERVICING 81

Thus, what is needed for the final stage of amplification
to produce great volume with a high degree of quality in
the course of reception of broadcast programs, Is a recti-
fier for supplying a high d.-c. voltage to a UX-210 type
tube, the latter being used in a properly designed stage of
audio-frequency amplification. This constitutes what is
termed a power amplifier.

POWER AMPLIFIER AND POWER SUPPLY
WITH FULL-WAVE RECTIFICATION

In Fig. 39 is shown the schematic wiring diagram of the
circuit arrangement for a power amplifier and power
supply, or B-eliminator, which is supplied with high-
voltage direct current from a full-wave rectifier circuit.
The following is a list of the material used by this writer
in the construction of the unit to be described, as well as
the circuit constants.

a—200-watt power transformer with two 10-volt secon-
dary windings, one 1,100-volt secondary winding with a
mid-tap, and one 110-volt primary winding.

b—Two 30-henry choke coils.

c—Audio-frequency low-ratio transformer, (2 to 1).

d—Output transformer, (1 to 1).

e—2-microfarad, 750-volt, filter condenser.

f—4-microfarad, 750-volt, filter condenser.

g—6-microfarad, 750-volt, filter condenser.

h—Four 1-microfarad, 200-volt, audio-frequency by-
pass condensers.

1—1-microfarad, 200-volt, grounding condenser.

4-—2-ohm, 2.5-ampere rheostat.

k—7-ohm, 1.25-ampere rheostat.

I—500,000-ohm potentiometer.

m—2,500-ohm variable resistance.

n—400-ohm potentiometer.

82 SRADIO CRECEIVERS

o—25,000-ohm heavy-duty resistor.

p—7,000-ohm heavy-duty resistor. -

g—8,000-ohm heavy-duty resistor.

r—10,000-ohm heavy-duty resistor.

s—25-ohm variable resistance.

t—Switch.

u—Two UX-216-B (or UX-281) tubes and sockets.

v—UX-210 power amplifier tube and socket.

w—Tell-tale lamp.

x—Two-single-contact jacks.

1 to 8—Terminals.

9 to 12—Terminals on double-terminal blocks.

The above material is the nucleus of a power amplifier
that is very satisfactory for reproducing broadcast pro-
grams. The 110-volt alternating current is supplied to
the power input receptacle through the medium of an
ordinary power plug. ‘The terminals of the input recept-
acle are connected to the primary terminals of the power
transformer, one of the leads in question being connected
through a single-pole single-throw switch ¢. The func-
tion of this switch is to turn the unit on and off. When
the amplifier is in operation, there is approximately 1
ampere drawn at 110 volts from the a.-c. supply line.
This means that the switch ¢ must have a current-carrying
capacity of at least lampere. Thus, the ordinary filament
switch will answer the purpose very nicely.

The rectifier filament-supply winding has a no-load
terminal voltage of 10, so a 2-ohm rheostat 7 is connected
in series with the filament supply to the rectifier tubes w.
These tubes are of the UX-216-B type and draw a filament
current of 1.25 amperes at 7.5 volts. The rectifier fila-
ment-supply winding must be insulated for voltages of the
order of 750 volts, for the mid-point of this winding is the
source of the rectified but unfiltered d.-c. supply. There is

AND SERVICING 83

alternating current at 550 volts (R. M. 8.) applied to the
rectifier plates, and the value of the rectified voltage is
of the order of 525 volts. The letters R. M. S. mean
root mean square, which in turn indicate that this voltage
value, which is indicated by an a.-c. voltmeter, is equal to

SitO ee isi2

v)

QQOQ orerere
2
A-C Power Y 1
Bl y2pU7 4 rhe
/ a

Fie. 39

the square root of the mean of the square of all the instan-
taneous values of voltage in one cycle of the a.-c. potential.

The effective value of the a.-c. potential is only .707 of
the maximum, or the peak, value that is reached in every
half cycle. ‘Thus, in order to calculate the maximum value

84 FRADIO SRECEIVERS

of potential that is applied to the plates of the rectifier
tubes, it is necessary to multiply by 1+.707, or 1.41.
This gives 550X1.41=775.5 volts, and this is the peak
value of voltage that is applied to the rectifier plates when
an a.-c. voltmeter indicates 550.

Now there is some drop in potential through the rectifier
tubes, and if this drop is 25 volts, the resultant will be
525 volts of rectified voltage. This rectified voltage, at
this point in the circuit is unfiltered; that is, it is not
smoothed out, and has the characteristics of an a.-c.
potential with the negative half of the cycle eliminated by

NS

a a

Fic, 40

the action of the rectifier tubes. With full-wave rectifica-
tion there is a pulsating potential that looks like an a.-c.
sine wave with all the negative half cycles inverted so that
they are on the positive side.

The changes that take place in the rectifier and filter
circuits are shown graphically in Fig. 40. Curve a shows
the characteristics of the applied a.-c. voltage. After

passing through the rectifier, which has unilateral impe- —

dance (allows the passage of current in one direction only)
the voltage has a pulsating characteristic as shown in
curve 6. This pulsating voltage is applied to the filter
circuit, which is composed of a large amount of inductance
and capacity that tend to keep the voltage from dying
down to zero in the course of its pulsations, owing to the

a

ENDS Ee RIV DCN G 85

charging and discharging of the condensers through the
filter chokes. The output voltage from the filter circuit is
thus of the nature shown in curve c. It is so close to a
d.-c. voltage that it can be termed such.

The high-voltage secondary winding on the power
transformer a, Fig. 39, has a potential of 1,100 volts
between its extremities, and there is a mid-tap on this
winding. The extermities of this high-voltage winding are
connected to the plates of the two rectifier tubes u. The
mid-tap of this winding is the source of the negative termi-
nal of the high-potential direct current and in this case is
connected to one side of all the filter condensers e, f, and g
and to the output terminal 5. Lead 4 is grounded through
the receiving set.

The source of the positive terminal of the mecunied but
unfiltered d.-c. supply is at the mid-point of the rectifier
filament-supply winding. A lead from this point is con-
nected to the first filter condenser e and to one end of the
first filter choke 6b. ‘The other end of the first filter choke 6
is connected to the second filter condenser f and one end of
the second filter choke. The other end of the second choke
is connected to the last filter condenser g and terminal q
of the double-terminal block. There is normally a jumper
between terminals 9 and 10.

Terminal 10 is connected to terminal 1/1 of the next
double-terminal block. It is also connected to the plate
of the power amplifier tube v through the primary winding
of the output transformer d. There is a jumper between
terminals 1/7 and 12. The function of the two double-
terminal blocks is to afford an expedient means of inserting
a milliammeter in the high-voltage supply circuit from the
rectifier to determine the following:

1. The total d.-c. drain on the rectifier.

2. The current to the plate of the amplifier tube v.

86 ‘RADIO CRECEIVERS

3. The direct current through the B-eliminator output

resistances.

4. The maximum voltage available at the output of

the rectifier filter circuit.

All of the above values may be determined by taking
two readings, one with the 50-milliampere milliammeter
connected to terminals 9 and 10, and a second reading with
the milliammeter connected to terminals 11 and 12. In
each case when the meter reading is taken the jumper is
removed from the two terminals to which the meter is
connected, and after the readings have been taken the
jumper is connected back again. The first reading men-
tioned, indicates the total direct current drawn from the
rectifier circuit. The second reading shows the current
through the B-eliminator output resistances 0, p, qg, and r.
It is to be noted that there should be no external con-
nections to the terminals /, 2, 3, 4, or 5 when the current
readings are being taken.

The difference between the two readings is the value of
the current to the plate of the power amplifier tube ».
The maximum voltage available at the filter output is equal
to the product of the current through the output resis-
tances, in terms of amperes, and the total value of the
output resistance, in terms of ohms, the product being the
voltage drop across the resistances in question, which is
the output voltage of the rectifier filter circuit.

It is very important to check, quite often, the current
from the rectifier circuit, as there is a great possibility of
overloading the rectifier tubes, or of passing too much cur-
rent through the filter chokes, or of operating the amplifier
tube with too great a value of plate current. The maxi-
mum d.-c. load current for a UX-216-B rectifier tube is
65 milliamperes and for a UX-281, 110 milliamperes.
This means that with half-wave rectification it would only

AND. SERVICING 87

be possible to draw 65 milliamperes from the rectifier cir-
cult of the UX-216-B and 110 milliamperes from UX-281
without overloading the rectifier tube. In full-wave
rectification the two tubes are operated in parallel to
supply direct current to the load, each tube supplying half
the load; therefore, with two UX-216-B tubes it is possible
to draw 130 milliamperes from the rectifier circuit without
overloading the rectifier tubes, and with two UX-281
tubes, 220 milliamperes may be drawn.

There is something else to bear in mind, and that is also
a limiting factor for the current in the case of full-wave
rectification, as well as the capacity of the rectifier tubes.
The standard types of filter chokes are designed to carry
up to 60 miliamperes, but beyond this point they are being
overloaded. ‘This overload is manifested in heat, which
may become of sufficient temperature to melt the insulat-
ing compound out of the chokes or burn out the wire.

Another point to bear in mind is the fact that the higher
the value of the current through the rectifier filter chokes,
the less the effective inductance, the greater the voltage
drop through this part of the circuit, and the less the avail-
able output voltage for the B-eliminator circuit and the
plate of the power-amplifier tube. Standard chokes for
this sort of circuit have a d.-c. resistance of the order of
600 to 1,000 ohms. If the minimum value of 600 ohms
be multiplied by 2, since there are two chokes in series in
the filter circuit, the effective d.-c. resistance will be 1,200
ohms. Now, if the current drain on the rectifier is kept
down to 30 milliamperes, the drop across the filter chokes
will only be .03 1,200, or 36, volts. But, if the current is
raised to 60 milliamperes, the drop across the filter chokes
is increased to 72 volts.

The UX-210 power-amplifier tube » with 500 volts on
its plate and a 45-volt bias, should draw about 30 milli-

88 RADIO RUE OU ERAS

amperes, and care should be taken to see that the bias is
sufficient to hold the plate current down to this value. The
plate of the tube will get hot when drawing 30 milliamperes
at 500 volts, because there is a plate dissipation of 500 x .03
=15 watts. The fact being kept in mind that the UX-210
is a 7.5-watt tube, it can be expected to show a little color
when it is made to dissipate 15 watts. An analysis of the
foregoing facts shows how important it is to have a milli-
amineter in series with the lead from the rectifier circuit.

As long as the rectifier and filter must be provided to
supply high-voltage direct current to the plate of the power
amplifier tube v, it follows that this supply might just as
well be made use of in effecting a B-eliminator circuit, since
the latter simply means the connecting of the proper resis-
tances across the rectifier output. In this case, the resis-
tances 0, p, g, and r function as the B-eliminator resis-
tances with a total resistance of 37,000 ohms. This
limits the current through these output resistances to
approximately 12 milliamperes. This is the current
through these resistances when there is no external load
to a receiving set. If the current from the rectifier circuit
is kept low enough it is possible to maintain a potential of
500, 135, 90, and 45 volts at the terminals /, 2, 3, and 4,
respectively, for supplying B-battery potential to a four-
or five-tube receiving set.

The condensers hf in the output of the power supply are
audio-frequency by-pass condensers across the plate supply
to the receiving set that is connected to the B-eliminator
terminals. If the power-amplifier B-eliminator unit is
located at a great distance from the receiving set, as in
another room, for example, the by-pass condensers in
question should be located right at the receiver.

The filament of the power amplifier tube v is supplied
with energy from a separate 10-volt winding on the power

AND SERVICING 89

transformer. This voltage is stepped down to the proper
terminal voltage (7.5) for a UX-210 tube by means of the
7-ohm rheostat k, which must have a current-carrying
capacity of 1.25 amperes, as this is the normal current
to the filament of a UX-210 tube at 7.5 volts.

The tell-tale lamp w is connected across the 10-volt
amplifier filament supply, in series with a 25-ohm resis-
tances. This little light is mounted on the front panel and
is a means of indicating when the amplifier is on or off.

The audio-frequency signal is supplied to the jack «x
through an ordinary radio plug. The terminals of this
jack are connected to the extremities of the primary wind-
ing of the input transformer c. The secondary terminals
of this transformer are shunted by a 500,000-ohm potentio-
meter / the contact terminal of which is connected to the
grid terminal of the power-amplifier tube v. This resis-
tance not only functions to aid the transformer in being
impartial to all the audio frequencies that are passed
through it, but it also functions as a volume control by vir-
tue of the fact that the grid can be connected to any point
along the resistance element of the potentiometer]. Maxi-
mum signal voltage is applied to the grid of the amplifier
tube when the potentiometer‘contact terminal is moved to
the upper end of the resistance element, and minimum
signal voltage when the contact is moved to the lower end.

The biasing resistance m is connected from the lower side
of the secondary winding of the input transformer c to the
mid-filament point, the latter being effected by means of
the potentiometer n whose extremities are connected across
the a.-c. filament supply, and the contact terminal of which
is maintained at the mid-point of the filament supply,
There is a by-pass condenser h across the biasing resistance.
The proper bias is applied to the grid of the power-amplifier
tube v by the drop in potential across the resistance m,

90 fRADIO “RECEIVERS

owing to the plate current to tube v through this resistance.
The resistance can be varied and in this manner the grid
bias can be changed.

It might be noted at this point that the two condensers h
(near m) and 7 are very important from a standpoint of
eliminating the hum from the signal output, and increas-
ing the volume as well as bettering the quality. When the

s
. es
S&
y (| bes
— ee
e

q Ey”
pm:

= = = LOTT
\eseai/ SS Ne :

aS

GS

y

TG

amplifier is in operation, note should be taken of the
amount of hum in the signal output with the power-supply
lead inserted in one direction. Then the plug in question
should be reversed and the hum again noted. Which-
ever way of inserting the power-supply plug produces the
least amount of hum in the signal output, is the way
which maintains the lead to which the condenser 7 is
connected, at ground potential. There is a decided

——

AND SERVICING gI

difference in the amount of hum in the signal output with
this condenser in or out of the circuit.

Shunting an audio-frequency by-pass condenser h
across the biasing resistance m increases the output volume
and makes a decided improvement in the quality of the
output signal. Shunting by-pass condensers from the
ends of the potentiometer n to the contact terminal makes
only a slight improvement, and they are not considered
necessary.

The signal output is taken from the secondary wind-
ing of the output transformer d through the jack z.

A rear view of the amplifier-eliminator unit is shown in
Fig. 41. The resistances and a few of the by-pass con-
densers can be seen mounted on the rear of the front panel.
The filter condensers and the B-eliminator output resis-
tances are mounted on the baseboard. ‘The power trans-
former is mounted at the right of the baseboard. The
tube sockets, filter chokes, double-terminal blocks, and
audio-frequency transformers are mounted on the sub-
panel.

POWER AMPLIFIER AND B AND C ELIMINATOR

In Fig. 42 is shown a schematic wiring diagram of a
power amplifier that is supplied with plate potential from
a half-wave rectifier unit. In this diagram, the detector
and two stagés of amplification are shown, with the con-
nections from the eliminator unit. The second audio-
frequency amplifier tube is the power-amplifier tube.

The following is a list of the constants, and the mate-
rial needed for the construction of the eliminator and the
two stages of audio-frequency amplification shown.

a—Power transformer with a 110-volt primary wind-
ing a, a 525-volt 60-milliampere secondary winding a,
and two 8-volt 2-ampere secondary windings a3 and ay.

b—Rectifier tube.

aa

‘RADIO SRECEIVERS

Tote wn nn nn ee en FF

92

AY G+&
O+S
= Gg

SI I—UL

S8LALS ff OL

AIYDIAS

saad Sa ple ee eae YS is a Se el a ed

AQEDIAGS ER LGUN G 93

c—Two 60-henry choke coils.

d—2-microfarad 750-volt filter condenser.

e—4-microfarad 750-volt filter condenser.

f—2- to 8-microfarad 750-volt filter condenser.

g—Heavy-duty resistor in six sections, having resistances
beginning from top of 9,000, 11,000, 4,500, 4,000, 3,500,
and 9,000 ohms.

h—100,000-ohm variable resistance.

1—Two 2,500-ohm variable resistances in series.

4—2,500-ohm variable resistance.

k—Detector tube UX-200-A.

[—Amplifier tube UX-201-A.

m—Power-amplifier tube UX-210.

n—Radio-frequency choke coil.

o—Audio-frequency choke coil.

»—Audio-frequency transformer (3 to 1).

g—Audio-frequency transformer (4 to 1).

r—60-henry choke coil.

s—.001-microfarad by-pass condenser.

i—Four 1-microfarad by-pass condensers.

u—2- to 4-microfarad fixed condenser.

v—two 1- to ;45-megohm grid leaks.

The biasing potential for the grid of the last-stage
amplifier, or power-amplifier tube m, is obtained through
the medium of the 2,500-ohm variable resistor 7. The
grid bias for the radio-frequency amplifier tubes is obtained
from terminal 1/0 by means of the two 2,500-ohm variable
resistors that are connected in series and designated as 7,
with two variable contact arms.

The potential to the plate of the detector tube k is
lowered below the value of potential available at tap T'-2
by means of a high-resistance variable unit h connected to
terminal 4. The other connections will be apparent from a
close inspection of the figure.

94 FRADIO SRECEIVERS

RECEIVERS WITH A.-C. TUBES

Desirability of A.-C. Operation.—In the operation of
radio receiving sets the trend has been toward complete
battery elimination. Many satisfactory plate-supply
units operating from an a.-c. source have been developed,
but filament operation from the same source has, for a
time, presented more of a problem because of the larger
currents required and increased expense in the rectifier and
filter circuits.

The development of tubes that used raw alternating
current in the filament circuit offered an excellent solution
of this problem. The grid and the plate circuits do not
offer any unusual problems. These are wired and con-
nected in practically the same way as in any set that is
operated by a B and C power unit. This discussion, will,
therefore, be focused on the filament circuit.

Wiring Receiver for A.-C. Operation.—The characteristic
features of the more common types of a.-c. tubes have been
given in another Section. The manner in which the fila-
ment circuits are wired is shown in Fig. 43. In the radio-
frequency stages and in the first stage of audio amplifica-
tion, radiotrons UX-226 are used. Radiotron UY-227
is used as a detector, and radiotron UX-171 or 171A asa
power amplifier. Potentiometers or center-tapped resis-
tors are connected across the filament leads to eliminate
the a.-c. hum.

A double-wave rectifier tube UX-280 is connected in the
high-voltage winding of the power transformer and the
output of this tube is used in the grid and plate circuits of
the receiver.

Grid Bias.—It is essential that the signal remain
undistorted as it passes through the various tubes of the
receiving set. In order to obtain quality reproduction the

5,

AUN@D 2, SsEeRey-L GIN’ G

‘oluny amg Olpny js/ A0f2ALIT

th “SI Boe oc ¢ AOU

edb te] te
Be Ee OF. re

1L!-Xf) 92e-KN L£EE-A/) 92e-X77 92C-X7 eile 17 t

66 FRADIO “RECEIVERS

proper B and C voltages must be used. One method of
biasing a five-tube receiver using a.-c. tubes is shown in
Fig. 44. The correct biasing voltages are obtained from
the output of the power unit. In the case of the detector
tube, the cathode is positive (43 volts) with reference to
the filament. This is done to prevent the filament from
attracting any of the electrons released by the cathode.

In the case of the power tube UX-171, the drop of
potential in the tube itself and between +C and —C gives
the necessary grid bias of —40 volts. The drop of poten-
tial between —B+C and —C is 45 volts, which is satis-
factory for the first audio tube. The radio-frequency
amplifier tubes require no biasing when the plate voltage
is not in excess of 67 volts.

SOUND REPRODUCERS
TELEPHONE RECEIVERS

Fundamental Form.—In its simplest form, the telephone
receiver, as shown in Fig. 45, consists of a thin, soft-iron
diaphragm P mounted close to but not touching one pole
of the permanent magnet NS. A fine wire ( is coiled
around one end of the magnet and the terminals of this
coil are connected directly in the circuit in which the
instrument is to be used.
The diaphragm is rigidly
supported at its outer edge,
but the center portion is
curved slightly toward the
magnet because of the at-
traction between the diaphragm and the magnet. If a
current is sent through the coil in such a direction that
the lines of force set up by it coincide with those of the
permanent magnet, the strength of the magnet will be
increased and the diaphragm will be drawn closer to the

AND? SERVICING 97

pole. If, however, a current is sent through the coil in
such a direction as to set up lines of force opposing those
of the magnet, the strength of the magnet will be dim-
inished and the diaphragm will spring away from the pole.

If a current that varies in value but is always in the
same direction is sent through the coil, the lines of force
induced in the magnet will increase while the current is
increasing, and decrease while the current is decreasing.
Thus, a varying pull on the diaphragm will cause vibra-
tions that will be in harmony with the changes in current,
whether the lines induced by the coil are in the same
direction as those of the magnet or not.

The telephone receiver is affected by the fluctuating
currents corresponding to sound waves and _ translates
these currents into distinguishable sounds. ‘The dia-
phragm of this simple device, like the diaphragm of the
reproducer of a phonograph, is capable of emitting the most
complex sounds; in fact, it is capable of imitating with a
fair degree of accuracy practically all of the sounds of the
human voice, of musical instruments, or other sounds
made up of many complicated wave combinations.

Standard Type.—A cross-sectional view of a standard
type telephone receiver is shown in Fig. 46. A barrel
or shell a is used to protect the component parts of the
receiver, and may be made of some insulating material

98 ‘RADIO CRECEIVERS

or in some cases is made of metal finished with an enamel
coating. The ear piece 6 screws onto the shell and serves
to cover the diaphragm end of the receiver except for a
small hole in its center through which sound waves are
permitted to escape. The permanent magnet c is U-
shaped, and has both poles projecting close to the dia-
phragm d to give as strong a pull as possible. The pole
projections carry the windings, which are equally dis-
tributed between the two coils and which are connected to

(a) Fic. 47

the terminals at e, only one of which is shown. Bringing
both poles of the permanent magnet close to the diaphragm
increases the number of lines of force effective on the
diaphragm and increases considerably the sensitiveness of
the receiving unit.

Watch-Case Type.—The construction of a compact
form of telephone receiver, known as the watch-case
receiver, is shown in Fig. 47; view (a) shows a section and
view (b) shows the end with the ear piece and diaphragm
removed. When the receiver is equipped with a head band,
as shown in view (a), it forms a head set, a name which is
applied whether one or two receiver units are used.

AND SERVICING 99

Although the principle of operation is the same as in the
larger hand receiver, the construction and design is
necessarily varied to decrease both the size and the
weight. The shell a consists of a case usually of metal,
threaded externally to engage an internal screw-thread
on the hard-rubber ear piece 6. The magnets are built
up of flat steel rings c, so magnetized that the opposite
sides of their circumferences are of different polarities.
The L-shaped pole pieces d, which reach nearly to the
diaphragm and carry the magnet coils, are attached to
the north and south poles of the steel rings. In many
cases the magnets are
not made of complete
rings, but are approxi-
mately half circles.
The extensions carry-
ing the coils are then
fastened to the ends of
the permanent mag-
net. The diaphragm
is of thin iron and is
clamped between the
body a of the shell and
the ear cap 0.

Radio Headset.—A
set using two watch-
case receiver units is shown in Fig. 48. This particular
type has a piece of soft iron mounted between the poles so
that it is acted on by their magnetic field. The coil of
wire carrying the received current is so located that it
moves the iron armature in a manner corresponding to the
current changes. ‘The armature is connected by levers to
a light diaphragm, often of mica, which is controlled by
the armature to produce sound waves in the air. The

Shee penton i
way mite —
—T

“sD
7727779

7:
99"

Fic. 48

100 ‘RADIO SRECEIVERS

lightness of the moving parts causes this receiver to be
responsive to very weak signals.

In some instruments of this general type an adjusting
screw is provided so that the sensitiveness of the signals
may be controlled to a certain extent. For instance, if it
is desired to weaken the signals, the armature is withdrawn
from the magnets by the screw arrangement. Also, the
tone or sound of the two receiver units may be exactly
balanced for best operation. If no screw adjustment is
furnished, the two receiver units should be adjusted at the
factory so that the tone of each is the same.

SPEAKERS

Horn-Type Speaker.—Where it is desired to make the
signals or message loud enough for a large group of people
to hear, it is necessary to
use a special type of
receiver unit known as a
loud speaker, or simply
speaker. The operating
unit of one type of horn
speaker is shown in cross-
section in Fig. 49, in which
the magnet coils are shown
at a and the metal diaphragm at b. The two coils are
mounted on extensions of a permanent magnet. The
diaphragm is in the form of a circular disk carefully
mounted between rubber gaskets and so suspended that its
surface rests but a small fraction of an inch above the
extension pole pieces of the magnet.

When a signal current passes through the coils, it causes
variations in the pull of the permanent magnet on the
diaphragm. Accordingly, the diaphragm vibrates, and
in doing so, sets up sound vibrations which emanate from

Fic. 49

ANID 3 SERVICING IOI

the mouth of the speaker. The horn, of whatever type
it may be, is attached to the piece c.

Cone-Type Speaker.—The principle of operation of a
cone-type speaker may be seen in Fig. 50. This speaker
consists essentially of a set of coils a, which when energized
actuate the armature 6. The armature is centered on the
bar c and is free to swing in either direction. ‘To one end
of the armature is fastened a drive pin, d, the other end of
the pin being connected to the thrust lever e, the connection
being made with soft solder. The thrust lever is, in turn,
connected to the apex of the cone f. Signal currents
flowing in the coils a cause the armature b to vibrate.
The vibration is trans-
ferred through the
pin d and thrust lever e
to the cone f. The
vibration of the cone
reproduces the music
or speech transmitted
from the broadcasting
station. No perma-
nent magnet is shown
in the figure; in actual
practice it is neces-
sary to have it.

Electro-dynamic Speaker With Power Amplifier.—In
Fig. 51 is shown in schematic form an electro-dynamic
reproducer a combined with a socket power unit containing
a stage of power amplification. B and C voltages are
also provided for the receiver that is used to drive the
speaker. ‘The reproducer a has two. coils. The coil b,
known as the field coil, forms part of the filtering system
of the power supply. A movable coil c, known as the cone
coil, inasmuch as it is rigidly fastened to the cone d,

So +

LO+tF

O6 +

ES Oi

f

SS ee ee ee eed

102

AND SERVICING 103

moves in the strong magnetic field of the coil b in accor-
dance with the modulation of the received signal.

The entire unit is designed to operate from a 105- to
125-volt 60-cycle supply. A two-way switch e is used to
regulate the input of the power transformer. Two UX-
281 radiotrons are used as rectifiers and are connected in a
full-wave rectifying circuit. The output of the rectifier
tubes is smoothed out by means of the two 4-microfarad
condensers f and the field coil 6 of the reproducer. The
filtered energy is applied to the plate of the power amplifier
tube UX-250 through the primary winding of the output
transformer g. The open terminals of the input trans-
former h are connected to the output of the receiving set
with which this unit is used. The secondary winding of
the output transformer g is connected to the cone coil c
of the reproducer a. The interaction between the field of
coil b and that of coil c produces a movement of the cone,
which, in turn, reproduces the broadcasted music or
speech.

SERVICING OF RADIO RECEIVERS

GENERAL INSTRUCTIONS*

CLASSIFICATION OF RECEIVING SETS

Classification of Circuits.—Instructions covering the
servicing of broadcast receivers are supplied by the manu-
facturer. These instructions, however, usually apply to
individual sets and are of little value in general application.
In this Section the service problems will be grouped under
general specifications, followed by methods of diagnosing
and correcting them.

In general, there are four basic pick-up circuits in use
today: the so-called regenerative detector, the untuned
radio frequency, the tuned radio frequency and the super-
heterodyne. Any set on the market may be classified as
using one of the foregoing types or possibly a combination
of one or more. There are two additional types of pick-up
circuits which have fallen more or less into oblivion and
will not be found in general use in the broadcast receivers
of today. They are the crystal detector and the straight
audion detector, which employs no form of regeneration
whatsoever.

Receiving sets consist of a pick-up circuit, a detector
circuit, and an audio-frequency amplifying circuit. In the
pick-up circuit radio-frequency amplification may be
incorporated. The detector may be either a tube or a
crystal. In the audio-frequency circuit from one to

*Abstract of article on “Servicing of Broadcast Receivers,” by
Lee Manley and W. E. Garity, in the Proceedings of the Institute
of Radio Engineers.

ACN Weta ER Vil CLIN, G 105

three tubes are generally used. In multi-tube sets employ-
ing radio-frequency amplifiers, some arrangement of
circuit is made to suppress or control the tendency of the
tubes to oscillate, when the circuits are tuned to resonance.
Any set on the market today may be grouped under one of
the foregoing classes as to the circuit employed.

Classification of Failures.—In much the same way that
receivers may be grouped under circuit classifications,
their failure to operate may be grouped under certain |
general classes, namely:

Lack of operating experience on the part of the user.

Location.

Defective accessories.

Open circuit.

Short circuit.

High-resistance connection.

Lack of operating experience may be the result of not
following out instructions carefully enough, or, as is some-
times the case, the instructions are not complete enough
and are not entirely clear to the novice. It may be the
result of insufficient instruction on the part of the service
man who made the installation. Then, too, it may be the
result of impatience on the part of the customer.

Under the caption of location many factors must be con-
sidered. The type of building in which the installation is
made, the proximity to steel buildings, power lines, trolley
and railway lines, and the geological and topographical
conditions surrounding the installation are all important
factors.

Under defective accessories may be included defective
tubes, power units, batteries, loud speakers, antenna and
ground installations, also improper battery connections.
Many sets fail or are returned to the dealer as unsatis-
factory because of poor antenna and ground installations.

106 CR ADIO RECEIVERS

Many a set of good quality and capable of delivering satis-
factory results fails because the loud speaker that is used
with it does not have the proper electrical characteristics
to operate satisfactorily in conjunction with the receiver.
Tubes will also cause trouble as they are subject to certain
defects incidental to fragility.

Open circuits are generally found in the movable con-
nections of the set such as a condenser pigtail, loop leads,
loud speaker leads, and any other connection that is subject
to movement or vibration in the normal operation of the set.
Open circuits may also result from burned-out transformers
or from mechanical failures in telephone jacks, rheostats,
and switches.

If a set has been once tested and found to be O. K., short
circuits rarely occur. When they do, it is the result of a
mechanical failure of the moving parts or of tinkering with
the mechanism of the set. It will sometimes happen that
the pigtail of a moving element of the receiver will break
and fall in such a way as to cause a short circuit of that
element. This is particularly true of the pigtails of varia-
ble condensers. The principal cause of short circuits that
occur in the normal operation of a set is in the tubes. If
the filament of the tube should break there is a possibility
of its falling in such a way as to cause a short circuit between
itself and the plate and grid elements of the tube. When
such a fracture of the filament occurs the B or C battery,
as the case may be, is short-circuited through the conduc-
tors involved. This type of short circuit is generally of
very brief duration as the filament will generally burn out
as soon as the short circuit occurs. A contact between the
grid and plate element of a tube is a more serious type of
short circuit, resulting in the rapid deterioration of the B
and C batteries, and may possibly cause a burn-out of the
transformer windings in the circuits involved.

AGN Diy E Revi Lic LG 107

The foregoing troubles are relatively easy to check up
as they are immediately apparent or can easily be located
by a continuity of circuit test.

The most difficult type of failure to locate is that caused
by a high-resistance connection. It is not only difficult to
locate, but it is difficult to determine. This condition
will cause the set to operate indifferently with rather
unsatisfactory results. This condition is sometimes mis-
taken as location trouble. A high resistance is possible at
any connection in the receiver. Soldered connections that
are soldered with a corrosive flux that has not been properly
treated after the soldering operation are probably the worst
offenders. Weak mechanical springs in telephone jacks
and switches may also introduce high-resistance connec-
tions.

| PRECAUTIONS

All sets should be tested before they are sold. This
takes but a few minutes and will surely pay well in avoid-
ing dissatisfaction as well as time that is sometimes neces-
sary to service a defective set. A radio receiver that is
working properly today does not as a rule go bad tomorrow,
and if such an installation does fail, the dealer may feel
that the trouble is due to a defective accessory rather than
the set itself. When the service man is called on to service
such a set he has the confidence that the set is O. K. and he
will immediately be able to concentrate on the real proba-
bility of failure rather than imaginary ones.

Then, too, if the dealer would acquaint the customer
with the limitations of radio reception, what to expect and
what not to expect, service problems would be minimized.
Acquaint the customer as to the probable length of time
his batteries and tubes will last. This is quite important,
and if followed out, will avoid some very disagreeable
service jobs,

4-8

108 ‘RADIO SRECEIVERS

SERVICE-SHOP EQUIPMENT

In order that satisfactory and efficient service may be
given to patrons, the radio dealer must have a properly
equipped service shop. The equipment should include
the following items: Long-nose pliers; combination
pliers; diagonal cutting pliers; 3-inch screwdriver; 6-inch
screwdriver; heavy-duty screwdriver; insulated screw-
driver; loud-speaker armature spacing tools; test leads with
springs clips; testing outfit with a.-c. and d.-c. meters;
a set of socket wrenches; 4- and 8-inch crescent wrenches;
small claw hammer; dust brush; electric soldering iron
and stand; spare-parts shelves; movable battery box or
power unit; indoor antenna; antenna terminal; ground
terminal; calibrated oscillator; headphones and cords; tool
rack; series test lamp with leads and test points; hand drill
with a selection of drills; small compass saw; small ball
peen hammer; wire; socket contact adjusting tool; friction
tape; rosin-core solder; polishing oil and cloth; pocket knife,
and any other instruments and tools that will enable the
man responsible for the service to do rapid and efficient
work.

PORTABLE TOOL KIT

From the standpoint of service it is usually advisable to
make all repairs on receiving sets directly on the owner’s
premises. In order to do this efficiently, the serviceman
must have a well-equipped tool kit, so that he may be able
to make all tests and repairs with as little delay as possible.
A well-stocked tool kit should have the following items:
Portable test set with a.-c. and d.-c. meters and the neces-
sary test leads; ear piece with head band; loud-speaker
armature spacing tools; set of tested tubes; long nose
pliers; diagonal cutting pliers; friction tape; electric solder-
ing iron and rosin-core solder; large and small screwdrivers :

ANDY VER VilO1N GC 109

insulated screwdriver; pocket knife; 4-inch and 8-inch
crescent wrenches; small hammer; large piece of canvas;
flashlight; and a selection of nuts, screws, wire, etc.

SERVICEMAN’S CONDUCT

When a service man goes into a customer’s home he is
usually going there as a representative of a commercial
establishment. He should be instructed to be courteous
and considerate. If he must take a set out of the cabinet
for adjustment he should use a piece of cloth provided in his
kit to protect the surface of the table he works on. He
should answer all questions asked him no matter how
absurd they may appear to him. The customer generally
has one question that he would like to have answered, and
in his mind the service man must be an expert, in order to
be able to do such work, and so he unburdens his mind.
The service man should respect this attitude on the part of
the customer and should do his best to point out the
fallacies tactfully and set the customer right in his ideas
about radio. The service man should make the customer
enjoy his visit and if this is done the service man becomes a
valuable asset to a business and is a potential salesman.

OBTAINING INFORMATION FROM CUSTOMER

The service man, before he starts to make any adjust-
ments other than turning on the set and trying the various
controls, should question the customer as to how it
happened, the time, place, and conditions surrounding the
failure. He should have the customer re-enact the con-
ditions at the time of failure. By getting all the symptons
an astonishing amount of time may be saved in running
down the difficulty. If sufficient questions are asked, the
customer will generally give the real cause of trouble or he
will suggest something in the course of inquiry that will
point out just what the cause of failure was. Sets as a

1IO SR ADIO SRECEIVERS

rule do not go bad of themselves. The failure usually
occurs while some operation is taking place, such as plug-
ging in the loud speaker, turning the condensers ormaking a
change in the battery connections.

The length of time that a set has been in operation will
be an indication of various types of trouble. <A set that
has recently been installed is subject to a certain type of
failure, whereas a set that has been in operation for a year
or more, 1s subject to other types of failure.

RELATION BETWEEN LENGTH OF SERVICE AND FAILURE

If a set has been installed for a period of two weeks or
less, outside of the inability of the customer to procure the
desired results, there are only a few reasons why the set
should fail. They are:

A defective tube.

Defective battery, battery connection, or power unit.

Loud-speaker connection loose in telephone plug.

Burn-out of transformer.

Of course, there may be other reasons, but these are the
most common and are given in the order of their probability
of occurrence.

If the set has been in operation for a period of six months
or a year, the possibilities of trouble will increase. If the
failure in this type of installation has been gradual, the
first thought would be that the tubes were becoming de-
activated through continual use.

If the breakdown was sudden, a mechanical failure
might be expected in one of the movable connections or
pigtails. A burned-out transformer could be expected in
difficulties of this sort. If the trouble is due to a noise
condition, the failure might be ascribed to dust or dirt
accumulations on the condenser plates or other important
parts of the receiver. The defect might also be due to a

AND SERVICING III

soldered connection. It will require, as a rule, a rather
long period of time for a soldered connection to corrode
to such a degree as to cause this condition. The local
atmospheric conditions under which the set has been
operating may have some bearing on the cause of failure.
If the set has been operating near the seashore and has been
subjected to the action of salt atmosphere it may have
caused sufficient corrosion of the connections or other
metallic parts to introduce high resistance or leakage
path.

If a set has been operating for a long period of time and
has given satisfactory results and then develops noises and
scratching sounds, one should not look for a loose connec-
tion in the wiring of the set, but rather look for an open
circuit in the moving parts. Worn mechanical parts are
often mistaken for loose connections in the wiring. The
wiring is absolutely stationary and it is not at all likely
that it will be disturbed in the ordinary use of the set so
as to cause a failure due to a loose connection. Vernier
drive shafts and vernier plates will wear loose, and while
apparently they are making perfect contact to the metal
surfaces of the condenser when the set is brought into a
critical condition, as is the case when receiving distant
stations, will cause noises that might be thought due to a
loose connection in the wiring.

PROBABLE SOURCES OF TROUBLE

TROUBLE IN ACCESSORIES

When trouble is experienced in radio reception the
general tendency seems to be to blame it on the set.
However, in a large number of cases the trouble may be
traced directly to a defective installation or to defective
accessories. This includes the aerial and ground, the A,

112 ‘RADIO SRECEIVERS

B, and C batteries or substitutes, the tubes, and the loud
speaker.

The aerial may be grounded; it may be too long or too
short; it may touch foreign objects; its connections may be
corroded; its lead-in may be broken inside the insulation.
The ground wire may be broken; it may be corroded at
connection to water pipe or other ground; there may be an
inefficient source of ground. The lightning arrester may
be leaky or short-circuited.

The A battery may be discharged. This is indicated
by a gradual dying out of the signals during reception.
The electrolyte should be tested with a hydrometer.
The connections at the terminals of the battery may be
corroded. This results in noisy, intermittent, or weak
reception. The terminals should be scraped bright and
then coated with vaseline to prevent further corrosion. If
an A-battery eliminator is used, the trouble will be found
either in the rectifier or filter circuit. In all such cases it
is best to report such trouble directly to the manufacturer.

Dry B batteries when run down cause reception to be
weak and noisy. When the detector voltage is too low,
the result may be a steady whistle. It is advisable to
replace a 45-volt battery unit when it drops to 34 volts.
The B-battery units may be wired incorrectly, resulting in
wrong voltages being applied to the plates of the tubes.

Storage B batteries have the same peculiarities as
storage A batteries. They should be tested periodically
with a hydrometer and inspected for corrosion at the
terminals.

The most common source of trouble in a B power unit
is a defective rectifier, especially if it is a tube. There is
also the possibility that the filter condensers have broken
down, the filter chokes or the resistors in the voltage
divider are shorted or open.

AND SERVICING 113

Defective tubes are another source of trouble. A tube
may light and yet be dead so far as reception is concerned.
Such a tube may sometimes be brought back to normal by
reactivation. Sometimes the elements are shorted, which
makes the tube unfit for reception. It happens also that
the wrong type of tube is used.

The loud speaker has also peculiarities of its own. It
may rattle, howl, or fail to produce any sound at all.
Replacing the loud speaker with a pair of headphones will
immediately determine whether the loud speaker or the set
is at fault.

OUTSIDE INTERFERENCE

Sources of Outside Interference.—In addition to the
complaints which the serviceman may find to be due to
faulty equipment or installation, there are also many cases
where unsatisfactory reception is due to interference
originating outside the installation. These interferences
manifest themselves in various ways. In order to deter-
mine whether a particular disturbance comes in from the
outside it is necessary to disconnect the aerial and ground
from the set and turn on the power. If the noise disappears
or is very much reduced in strength, then the disturbance
is undoubtedly due to something outside the set. Among
the many outside sources of interference may be mentioned
static, nearby oscillating receivers, radio telegraph sta-
tions, and defective power installations.

Much of the work in mitigation of electrical interference
results in an improvement in the operation of the electrical
devices or supply lines and is thus a double gain. There
are, however, some electrical devices which, even when in
perfect working order, cause disturbances that result in
interference with radio reception. In many cases it is
possible to provide filters, shields, chokes, etc., either at

114 SR ADIO CRECEIVERS

the source of disturbance or at the receiving set, which
do much to relieve the difficulties.

Part of the disturbance from electrical devices is prae-
tically inevitable and must be regarded, like atmospheric
disturbances, as part of the inherent limitation of radio
reception. In other words, the limitation upon radio
reception is not only the distance and the power of the
transmitting stations and the sensitiveness of the receiv-
ing set, but also the omnipresent background of slight
electrical disturbances which drown out signals below a
certain intensity. This background of electrical dis-
turbances is the underlying reason why reception from
local stations is inherently superior to reception from dis-
tant stations.

Power-Line Induction.—A frequent cause of interference
is the presence of alternating-current power wires near
the antenna or receiving set. Low-frequency voltages
(usually 60 cycles) are induced and the resultant current
flowing in the receiving circuit causes a “humming” sound
in the telephone receivers. The low pitch of the hum will
usually identify this source of interference. A method of
eliminating or at least reducing the magnitude of this
interference is to place the antenna as far as possible from
the wire lines and at right angles to them. When the
interference can not be eliminated by such means, the
proper choice of a receiving set may help. An inductively-
coupled (two-circuit) receiving set is less susceptible to
such interference than a single-circuit set. The use of one
or more stages of radio-frequency amplification should
also help to filter out the audio-frequency interference.
It has been suggested that audio-frequency interference
might be shunted around a receiving set having a series
antenna condenser by connecting between the antenna and
ground terminals of the set a high resistance, which will

AND SERVICING Il5

offer lower impedance to the audio frequency than will the
receiving set itself.

Sparking Apparatus.—Sparks are produced in the normal
operation of many types of electrical apparatus, such as
motors, doorbells, buzzers, gasoline engines, X-ray
apparatus, violet-ray machines, some forms of battery
chargers, rural telephone ringers, and heating-pad thermo-
stats. Sparks are also sometimes produced at defective
insulators, transformers, ete., of electric wire lines.
Sparks usually give rise to electric waves which travel
along the electric power wires and by them are radiated
out and are then picked up by radio receiving sets. The
noise thus produced in a radio set may come from a
disturbance that has traveled several miles along the
electric power wires.

One remedy for such types of interference is to eliminate
the spark. This is possible if the spark is an electrical
leak and not necessary: to the operation of the machine in
which it occurs. Many very useful electrical machines,
however, require for their operation the making and break-
ing of electrical circuits while they are carrying current,
and whenever this happens a spark is produced. It is
impossible to eliminate these machines, so that it is neces-
sary to make the spark of such nature or so arrange the
circuits that the radio-frequency current is reduced or
prevented from radiating.

To prevent the radio-frequency current produced by a
spark from getting on to the lines connecting the sparking
apparatus, some form of filter circuit is necessary. A
tested condenser, 1 microfarad, more or less, connected
across the sparking points will short-circuit a considerable
amount of the radio-frequency current, or a condenser
connected from each side of the line to ground will serve
the same purpose. A choke coil in each side of the line in

116 ‘RADIO CRECEIVERS

addition to the condensers connected to ground forms a
simple filter circuit that should prevent frequencies in
the broadcast range from getting on the line. A high
inductance, or choke coil, or a high resistance connected
in each side of the line changes the characteristics of the
circuit so as to reduce the amount of power radiated. If
such a filter circuit is not effective or is impractical, the
apparatus may in some cases be surrounded by solid
metal sheet or wire screen that is thoroughly grounded.
The screen should completely surround the apparatus.
This may be difficult. For example, in shielding the igni-
tion system of a gasoline engine the spark coils and all
wires and other parts of the system must be enclosed in
metal shields, and these must be very well grounded.
Location of Source of Interference.—The first thing to
do in tracing the source of trouble is to make sure that it
is not in the receiving set itself. The next thing is to open
the electric switch at the house meter; if the interfering
noise is still heard in the radio set, the source is then known
to be outside the house. It is then desirable to report the
situation to the electric power company. Many of the
companies have apparatus for the purpose of following up
complaints of this kind. Usually a sensitive receiving
set with a coil antenna is used to determine the direction
from which the interfering noise comes, and this outfit is
taken from place to place until the source is found. The
location of such sources is often a very difficult and baffling
undertaking. The trouble sometimes comes from a spark
discharge over an insulator to ground, or between a pair of
wires, or it may be that the wire is touching some object -
such as a tree, pole, guy wire, etc. Such a spark discharge
is a loss of power to the operating company and a potential
source of serious trouble, and for these reasons the company
is probably more interested in finding and eliminating

AGNES REV OLN: G 117

this type of trouble than the radio listener. Large leaks
and sparks may often be observed at night, especially in
hot weather. However, sparks that are too small to be
readily noticed may cause serious interference to radio
reception.

Commutators.—Where d.-c. motors are in operation
near a radio receiving set interference is sometimes caused,
especially when the brushes on the motor are sparking
badly. The sparking should be reduced as much as
possible by cleaning the commutator and by proper setting
of the brushes. The remaining interference is sometimes
overcome by placing two condensers, about 2 micro-
farads each, in series across the power-supply line and con-
necting their mid point to a good ground system.

Bell Ringers.—Another source of interference is the
ringing machine used in rural telephone exchanges. Tele-

phone engineers can reduce or eliminate interference by
connecting a filter between the machine and the ringing
keys.

Precipitators.—Many cases of radio interference have
been caused by electrical precipitators that are used to
prevent smoke and noxious fumes or material from leaving
the chimney. The precipitator operates by establishing a
highly charged electric field, inside the chimney, of such a
nature and direction that particles going up the chimney
are charged and driven against the walls where they stick.
Precipitators cause interference for the reason that the
high voltage used in their operation is obtained from a
rectifier that produces sparks and generates radio-fre-
quency alternating current as well as the direct current
which the precipitators need. If the precipitator is so
designed and arranged that the distance between the
rectifier and the chimney is only a few feet or if the entire
apparatus including all leads is housed in a metal build-

118 SR ADIO SRECEIVERS

ing, there is usually no trouble. But if the rectifier is
separated from the chimney the wire which joins them
forms a good antenna that will radiate and cause inter-
ference for 20 miles or more. Interference from these
precipitators can be eliminated by placing a grounded wire
screen entirely around these wires and thoroughly ground-
ing the wire screen and the rectifier. If screening of the
various parts is impracticable, damping resistances can be
inserted at various points in the wire line, which will
reduce the amount of power radiated. Tuned circuits
connected across the spark gap of the rectifier will assist
by absorbing the radio-frequency power.

TESTING OF RECEIVING SETS
WESTON A.-C. AND D.-C. TESTER

Purpose of Tester.—The Weston a.-c. and d.-c. tester,
known as model 537, is designed for testing any type of
receiving set, whether operated from a direct-current or an
alternating-current source. It will measure the various
voltages used in the radio set; it will test continuity and
condition of circuits, and test the tubes under the same
conditions as exist when the tubes are in their sockets.
All tests can be made by using the regular voltages
normally supplied to the set by its batteries or socket
power, so that no auxiliary power supply is required.

A.-C. Voltmeter.—The test set, shown in Fig. 1, has
two instruments, an a.-c. voltmeter a and a d.-c. volt-
milliammeter b. It is provided with various switches, plugs,
binding posts, cords, and adapters for properly connecting
the instruments to the circuits under test.

The a.-c. voltmeter a has three ranges ; hamely, 150, 8,
and 4 volts. Hither of the two lower ranges is connected
directly across the filament terminals when the switch cis
set to A.C. The particular range depends on the position

AND SERVICING 119

of the range selector switch d, whether turned to 8 or 4,
or away or toward the operator, respectively. These
ranges are for the purpose of measuring the filament volt-
ages of tubes when the filaments are heated with raw
alternating current. This voltmeter may be allowed to
remain in the circuit during the tests for plate voltage,
plate current, grid-bias voltages, and tube tests described
later. This will enable one to follow any changes that
may occur during tests due to changes in line voltage.
The 150-volt range is provided for measuring the line
voltage and is available only at the two binding posts e.
These are marked 150 and + on the instrument. This
range is entirely insulated from all other circuits in the test
set, and, therefore, while only one range can be used at a
time to obtain correct readings, no damage to the set can
result if both high and low ranges are connected simultane-
ously, and it may remain in the circuit during any other
test, regardless of connections, without damage or error.
The low ranges are also available at the three binding
posts f; these are stenciled 8, 4, and X. The low-range
binding posts must not be used when the plug g is connected
in the radio set, on account of possible interconnections.
D.-C. Volt-Milliammeter.—The d.-c. volt-milliam-
meter 6, Fig. 1, has four voltage ranges; namely, 600, 300,
60, and 8 volts, and two current ranges, 150 and 30 milli-
amperes. The 600- and 300-volt ranges are for plate
voltage, the 60-volt range for grid bias, and the 8-volt
range for filament voltage measurements. These ranges
are all available at the plug g for tube-socket tests, by
properly setting the dial switch h. The 600- and 60-volt
ranges are also available at the three binding posts 7
shown on the right of the instrument, when the dial
switch h is set to Vm. B. P. (voltmeter binding posts). All
volt ranges have resistances of 1,000 ohms per volt, so that

120 Ravio SRECEIVERS

they may be used for measuring the voltages from socket
power devices.

The 30- and 150-milliampere ranges are available at the
plug for plate-current measurements by setting the dial
switch to Plate MA., and the range selector switch 7 away
from the operator for 150 milliamperes, and toward the

oo
i 4
ONT RAS

operator for 30 milliamperes. The 150-milliampere range
is provided for measuring higher plate currents than 30
milliamperes and the output of rectifying tubes. The
600-, 300-, and 8-volt ranges may be read directly on the
scales provided for them. The 60-volt and 30-milli-
ampere ranges should be read on the 600 and 300 scales,
respectively, dividing the indications by 10. The 150-

AND aj5 ERVECEN G I21

milliampere scale should be read on the 300 scale divided
by 2. The 150-milliampere range is also available at bind-
ing posts k by setting the dial switch h to MA. B. P.

Dial Switch.—The dial switch h, Fig. 1, is a bipolar
switch, and connects the d.-c. volt-milliammeter 6 to the
various circuits designated on the dial. ‘Two positions are
available for plate-voltage tests, namely, 600 and 300 volts.

A.-C.—D.-C. Switch.—When the switch c, Fig. 1, is set
toward the mark A. C., or to the left, it connects the low
ranges of the a.-c. voltmeter a across the filament terminals
of the plug g for measuring filament or heater voltages of
tubes supplied with raw alternating current. When the
switch is set to the mark D. C., or to the right, the low
ranges of the a.-c. voltmeter are disconnected and the
8-volt range of the d.-c. voltmeter 6 can be used for fila-
ment voltage measurements of tubes supplied with direct
current or rectified alternating current.

This switch has only the one function referred to, and all
tests, either for a.-c. or d.-c. tubes, except filament’voltage,
are made on the d.-c. meter, including plate and grid-
bias voltages and plate current, regardless of the position
of this switch. When d.-c. filament voltage are to be
measured, the switch c should be set to D. C. for the reason
that both instruments will indicate and the a.-c. meter
requires more current for its operation than the d.-c.
meter, which may cause an error in the voltage indications.
No harm, however, can result if the switch cis left on A. C.

Binding Posts.—Binding posts are provided for making
voltage or current measurements directly on batteries or
power units, or for any purpose for which the cord and
plug are not adapted, as, for example, testing the heater
voltage of tubes when the heater terminals are not in the
base of the tube. The plug should be removed from the
radio set when the binding posts are used.

122 RADIO SRECEIVERS

Adapters.—The adapters are shown in the foreground
of Fig. 1, on the right and left of the plug g. The set is
provided with a plug and socket of the UX type. If the
set 1s equipped with tubes or sockets other than the UX
type it is necessary to select the proper adapters to make
the test. A plug with cord and terminals attached is
also provided for connection to a lamp socket for line-
voltage tests.

TESTING BATTERY-OPERATED SETS

Preliminary Adjustments.—First see that the A, B, and
C batteries are connected to the radio set and all tubes in
place except in the socket to be tested. Then insert the
plug g, Fig. 1, into the empty socket in the set, and the
tube into the socket J of the tester. Set the switch c to
D.C. and the switch 7 to 30 and proceed with the tests.
In new radio sets not previously tested, before any tubes
are inserted it is preferable to make a preliminary test of
each socket in succession. This will reveal defects, if
any, in manufacture and those due to shipment, and
possibly save tubes.

In radio sets having phone or speaker vee it is neces-
sary to plug in either a headphone or speaker before tests
can be made on certain tube sockets, especially in the last
stage, as the plate voltage is connected to the plate of the
last tube through the phone circuit. In some sets the
filament voltage also can not be applied to a tube until a
plug is inserted in the corresponding jack.

Testing A Battery.—Turn the switch dial h, Fig. 1, to
A or A Rev., whichever gives a positive indication, and
adjust to the filament voltage for the tubes, using the
8-volt scale on the instrument. If no indication can be
obtained on the instrument, then there is either a broken
or disconnected circuit or the A battery is entirely run

AND SERVICING 123

down. If a slow indication results, then either the
A battery is low or the contacts are poor in the sockets,
or in the rheostat, or in the circuit connections. If an
unsteady indication results, then some connection is loose.
Loose or variable contacts are frequently found in the
spring contacts in the tube sockets and in rheostats.

In the above tests, the voltmeter indicates the voltage
across the filament. In order to measure the total A-bat-
tery voltage from the socket it is necessary to remove all
the tubes from the set and tester and set the rheostat at
or near its maximum position. Then the voltmeter indi-
cates the A-battery voltage directly. With storage A
batteries a more reliable test is obtained with a hydrom-.
eter.

Testing B Battery.—Turn the switch dial h, Fig. 1, to
either B 300 or B 600, depending on the voltage to be
measured, and then read the indication on the correspond-
ing scale. If the circuits are good, this indication will
give the B-battery voltage less the drop in the primary
of the transformer. For radio-frequency transformers this
drop is negligible and for the average audio-frequency
transformer, on account of the high sensitivity of the
instrument, the indicated voltage will be within 1 per cent.
of the actual B-battery voltage.

If no indication results, the following should be looked
for:

(a) Disconnected battery.

(6) Run down battery.

(c) Spring contacts in socket out of place.

(d) Open circuit in primary of transformer.

(e) Open connection in some part of the plate circuit.

If low indication results, then look for the following:

(a) Partly run down battery

(6) Loose or corroded connection.

4-9

124 INASD VOPe RE cr ievenenes

If a variable indication results, then look for a loose
connection in some part of the circuit.

Testing C Battery.—Turn the switch dial h, Fig. 1, to C
or C-A Rev.; depending on whether it was necessary to use
A or A Rev., respectively, when testing the A battery.
The C-battery voltage is indicated on the 60-volt range
and is read on the 600-volt scale divided by 10. The
voltage read will be less than the actual C-battery voltage
by the amount of the voltage drop in the secondary of the
transformer. For radio-frequency circuits, when no grid
resistors are used, this drop is negligible. For audio-
frequency transformers the voltage indicated will be
about 90 per cent. of the battery voltage. The C-battery
circuit should be tested in the same manner as the B-bat-
tery circuit, it being remembered that the secondary of
the transformer, instead of the primary, is in circuit.

Locating Circuit Defects——When a defective circuit is
indicated in the foregoing tests, a simple method for locat-
ing the defective part is to remove the plug g, Fig. 1,
from the radio set, and connect the two cables furnished
with the set to the binding posts on the instrument, one to
— and the other to 600, the switch dial h being set to
Vm. B. P. Then if the B-battery circuit is to be tested,
the free end of the cable that is connected to the — bind-
ing post should be connected to the negative terminal of
the B battery on the radio set, and with the free end of the
remaining cable, the plate terminal or contact spring in the
socket should be touched where the defective circuit was
indicated. This circuit should be followed from one
connection to another. When the defective part has been
passed, the voltmeter will give-an indication, showing that
the defect is in the part of the circuit just passed.

The A- and C-battery circuits can be tested in the same
manner as described for the B-battery circuit. It is

AN Ds SERV. UCUNG 125

better to use the high-voltage range for all of these circuits
for the reason that if-a high-voltage circuit is touched
accidentally when tracing a low-voltage circuit, then no
damage to the instrument or to the radio set will result.

Testing Batteries Directly.—To measure the voltage of a
battery or battery substitute directly at its terminals,
connect them to binding posts — and 60 or — and 600 on
the tester, Fig. 1, depending on the voltage to be measured.
Turn the switch dial h to Vm. B. P. This connects the
binding posts to the instrument and makes it an ordinary
double-range voltmeter.

Testing of Tubes.—Remove a tube from the radio set
and place it in the socket J, Fig. 1, on the tester. Insert
the plug g into the socket from which the tube was removed.
_ For comparative tests of ordinary tubes it is desirable to
select a socket having a B battery of about 90 volts and a
C battery of 4.5 volts. If other sockets are also controlled
by the same rheostat that controls the socket selected for
the test, then these sockets must also contain tubes.

Power tubes, such as UX-120, UX-112A, UX-171A,
UX-210, and UX-250 preferably should be tested from the
socket in which they are regularly used on account of the
higher voltage B and C batteries require.

If it is desired to test all the tubes directly from the
sockets in which they are to be used, then all the tubes
may be in place in the radio set except the one to be tested,
and the plug in the tester inserted in the socket belonging
to this tube. Then, by interchanging the tubes in succes-
sion, all can be tested. In this test it is preferable not to
use the detector-tube socket.

The first test is to determine if the filament or grid is
touching the plate. This is accomplished as follows:

1. Set the switch dial h, Fig. 1, to B and then to C0
to make sure that the B and C batteries are connected and

126 RADIO SRECEIVERS

are of the correct values. The filaments may be lighted
or not as is found necessary or convenient.

2. Then set the switch dial h to Plate MA., and the
range selector switch 7 to 30, and insert the tube to be
tested into the socket J. If the pointer on the instrument
deflects violently to the right beyond the scale, it indicates
that the filament or the grid is touching the plate. The
tube should be immediately removed from the tester.
In testing power tubes, the plate current resulting when
tested without the proper grid bias will often be greater
than the full-scale value on the instrument. This com-
paratively mild slamming of the pointer must not be
mistaken for the violent slamming resulting from a defec-
tive tube.

If, after the foregoing tests, the plate-current test
indicates approximately normal values on the scale, then
the filament and grid are not in contact with the plate and
further tests should proceed as follows: Set the switch
dial h to A or A Rev., as is found necessary, and adjust the
filament voltage, for which the tube is designed by means
of the proper rheostat in the receiving set. Change the
switch dial h to B and read the B-battery voltage. It is
necessary that all tubes be tested at the proper plate
voltage, in order to obtain comparative readings. Then set
the switch h to Plate MA., and the range-selector switch j
to 30 or to 150, as is found necessary. This changes the
instrument into a milliammeter having a full-scale value
of 30 or 150 milliamperes. Read the plate current on the
300 scale divided by 10 for the 30-milliampere range, or
divided by 2 for the 150-milliampere range.

To determine whether the grid of the tube is in operat-
ing condition and to indicate roughly the condition of the
tube as an amplifier, set the dial switch h on Plate MA.,
the range switch 7 at 30, and press the key m. When the

SAND. SERVICING 127

key m is up in its normal position the grid is connected to
the C battery in the set, and the current indicated on the
instrument is the normal plate current of the tube. When
the key m is depressed, the grid is connected to the —A
terminal, which gives zero grid voltage, with a consequent
change in plate current. If the grid is functioning
properly, the plate current will be increased upon pressing
the key, and the increase in current, when properly inter-
preted, is a rough measure of the condition of the tube as
an amplifier.

If no change in plate current occurs upon pressing the
key, then the following may be the cause:

1. The radio set has no C battery.

2. The C battery, if used, is run down or disconnected.

3. The grid connection may be broken in the tube, or
the grid. may be touching the filament. In either case the
tube should be replaced. The approximate plate current
values for the different types of tubes are given in the
printed circulars that accompany the tubes.

To measure the total current drain on the B battery
connect the 150-milliiampere binding posts k on the tester
in series with the battery circuit at the —B terminal and
set the dial switch h to MA. B. P.

TESTING A.-C. OPERATED SETS

Sets Using Raw A. C. for Filament Heating.—All radio
sets, whether a.-c. or d.-c. operated, must have direct
current for the plate and grid circuits. Therefore, if a.-c.
operated, the plate and grid voltages must be obtained by
rectifying and filtering the alternating current by means
of suitable power units. All tests on plate and grid voltages
and plate current must be made on the d.-c. volt-milliam-
meter b, Fig. 1, just as on sets operated by batteries or
battery substitutes. |

128 SR ADIO SRECEIVERS

The filaments on a.-c. operated sets may be heated either
directly by raw alternating current or by rectified and
filtered alternating current. To test a set equipped with
tubes using raw alternating current, the switch c should be
set to A. C.; care should be taken to see that the power
supply is properly connected to the set and that all tubes
are in place except in the socket to be tested. Then the
plug g should be inserted into the socket of the set and the
tube into the socket /. Then the same procedure should be
followed as is described for battery-operated sets, except
for the following: |

Read filament voltages on the 4-volt or the 8-volt range
of the a.-c. meter a, and all other voltages and currents on
the d.-c. meter b, using the dial switch h as previously
described. A low or no reading means a defect in the
power unit or in the connections.

To test the five-prong U Y-227 detector tube by voltages
from its own socket in the radio set, two adapters must be
used; one to adapt the four-prong plug g to the five-hole
socket in the set, and the other to adapt the five-prong
tube to the four-hole tester socket J. The plug adapter is
so designed that the cathode circuit in the radio-set socket
is not connected to the tester. The heater current is
supplied to the heater element in the tube through the
usual filament connections in the plug g, and the cathode is
connected to one of the filament terminals in the tube
adapter. With this connection, the plate voltage, fila-
ment voltage, and plate current as they exist when the tube
is in use are measured as for any other tube, but since no
C voltage is available in the detector socket, a definite
grid test cannot, in general, be made under these conditions.

As the currents required for the UY-227 and UX-226
tubes are comparatively large, there will be a slight drop in
the connecting leads when their voltages are being tested.

ANDI SERVICING 129

To obtain the true values of the filament voltages at the
transformer terminals add .16 volt to the indication of the
U Y-227 tube and .1 volt to the UX-226 tube.

Sets Using Rectified A. C. for Filament Heating.—Sets
using rectified alternating current for heating the filaments
may be divided into two general classes; namely, those in
which the filaments are connected in parallel and those in
which the filaments are in series. The sets in which the
tubes are connected in parallel are tested in the same way
as battery-operated sets.

In series-filament operated sets it is preferable to select
a socket having about 90 volts plate potential, and con-
nected in the radio-frequency or intermediate-frequency
circuit, depending on the type of radio set. Insert the
plug g, Fig. 1, into this socket, and the tube into the
socket | of the test set. For the complete test the other
tubes should be in their respective sockets.

Set the switch h to A or A Rev., as is found necessary to
give a positive deflection. Note the voltage, which is the
voltage across the filament. Then test for plate current,
plate and grid voltage and make the grid tests as for bat-
tery-operated sets. Try each tube in succession, removing
each one from its socket and placing it into the socket on
the tester, remembering to insert the tube just tested into
the vacant socket.

If all the tubes are equally low in filament voltage, it is
possible that the trouble lies in the power supply, or that
in one or more of the tubes the grid is touching the fila-
ment. This can often be discovered by gently tapping the
tube. If, however, voltages differ among the tubes, then
the fault is most likely to be in the tube circuits. While
changing tubes in making this test it is preferable to have
the power supply shut off to prevent possible excessive
voltage from being applied to other tubes.

130 FRADIO SRECEIVERS

No hard and fast rules can be given for these tests, as
the sets differ so much in their construction and type of
circuits. Anyone, however, familiar with the circuitsof any
one particular set can work out methods for using the
tester, by following the general directions given herein.

The foregoing instructions have been prepared by the
Weston Electrical Instrument Corporation and apply
directly to the use of the test set just considered. These
same instructions, however, are applicable, with certain
modifications, when testing radio receivers with the
ordinary a.-c. and d.-c. meters.

SPECIFIC TROUBLES AND ADJUSTMENTS

SIMPLE TESTING EQUIPMENT

Continuity Tester.— When a more elaborate instrument
is not available, the simple arrangement shown in Fig. 2
may be used to test the continuity of circuits, windings of
transformers, coils, etc., or to locate defective condensers,

FiGa2

short circuits, and grounds. The tester consists of a pair
of headphones a, a 43-volt battery b, and the test points c.
In place of the headphones one may use a voltmeter with
voltage sufficient to give a full-scale deflection when con-
nected directly across the battery terminals. The use of
the voltmeter is very convenient in checking the voltage
drop in the circuits of a receiver. The intensity of the
click in the phones or the indication of the voltmeter,
whichever may be used, shows approximately the condition
of the circuit under test.

AND SERVICING 131

Resistance Measurement.—In a large number of cases
the serviceman is confronted with the problem of determin-
ing the values of resistors used in receivers and power units.
According to Ohm’s law, the resistance R of a circuit,
in ohms, is equal to the electromotive force E, in volts,
divided by the current J, in amperes, or

h=L=]
When the current is given in milliamperes, the formula
becomes ee 1,000H + T

From the foregoing one can justly reason that the
resistance can be very readily calculated when the electo-
motive force and the current are known. A simple scheme
for measuring these quantities is shown in Fig. 3. The
measuring unit consists of a 6-volt battery a, a 30-ohm

TGA

rheostat b, a voltmeter V having a working range from 0 to
8 volts, a milliammeter MA with a scale of 1 to 250 milli-
amperes, and a pair of test points c, all connected as shown
in the figure. To determine the voltage and current
values, the test points c are placed on the terminals of the
unit the resistance of which is to be measured and the
rheostat adjusted until satisfactory readings are obtained.
The resistance is then calculated as explained in the preced-
ing paragraph.

Modulated Oscillator——For certain radio adjustments
it is necessary to have a source of modulated high-fre-
quency energy to energize the radio-frequency circuits of

4—10

132 FRADIO SRECEIVERS

the receiving set and produce an audible note in the phones
or in the loud speaker. The most satisfactory generator
of high-frequency energy is a vacuum-tube oscillator, a
convenient type of which is shown in Fig. 4. The coil-
condenser combination, L and C, respectively must be
designed to cover the frequency range of the receiving set.
For broadcast receiving sets the approximate frequency
range is from 500 to 1,500 kilocycles.

The coil to be used with a .0005-microfarad tuning
condenser to cover the broadcast range may be wound on

=I

Fic. 4

a 25-inch bakelite tube with fifty turns of No. 20 double-
silk-covered wire. The coil is tapped at the twenty-fifth
turn and a connection made to the negative terminal of the
A battery.

Type UX-199 tube is recommended for the oscillator,
although the general purpose tube UX-201A may also be
used. With a UX-199 type tube, a 44-volt C battery may
be used to light the filament, or act as the A battery. A
30-ohm rheostat is used in the filament circuit. Two
small 223-volt B-battery units may be used in the plate
circuit. In this way it is possible to make the oscillator a
self-contained unit.

The grid condenser may have a capacity of .00025
microfarad. The resistance value of the grid leak deter-
mines the pitch of the audible note produced by the oscil-

AIN'D® J fiRV ECIN.G gk.

lator. A 4- or 5-megohm leak will probably give the
desired tone. If a higher tone is desired, a lower value
of grid leak is used, and vice versa.

VARIABLE-CONDENSER TROUBLES

Possible Troubles.— Variable condensers are the tuning
units in most of the present-day radio receiving sets.
They are thus exposed to a lot of wear and tear, and, unless
ruggedly constructed and well mounted, they will in time
cause considerable trouble and inconvenience. The dials
may slip out of position; the movable contacts, when not
protected by pig-tail connections, may become dirty or
loose, and introduce noises; the plates may become
covered with dust, which also results in noises and lessened
efficiency; the plates may be bent or the entire rotor or
stator laosened, so that the condenser is shorted through
part or the entire tuning scale; one of several condensers,
when all are controlled by one tuning dial, may slip out of
position and detune the entire assembly for all wavelength
settings. The remedies for some of these troubles are
quite obvious. The location and adjustment of many of
these troubles, however, are not easy and should not
be atempted by any one but those familiar with the correct
procedure.

Shorted Plates.—Shorted plates are in some cases evi-
denced by a scraping noise when the condenser dial is
turned. In most cases, however, it is necessary to
test the condenser electrically to determine the nature of
the trouble. The simple test set shown in Fig. 2 may be
used for this purpose. Disconnect the condenser from its
circuit and connect the test points c to the terminals of the
condenser with the rotor plates entirely out of thestationary
plates. A short circuit in the plates will be evidenced
by a click in the phones when the condenser dial is turned.

134 CRADIO “RECEIVERS

A visual inspection will then reveal the difficulty. Bent
plates can sometimes be straightened with the ordinary
long-nose pliers. Wear at the bearings can be com-
pensated by tightening the adjustments, when such are
provided.

Body Capacity.—Sometimes the tuning of a set is affected
by the operator’s hand in contact with the condenser dial.
This is known as body capacity. The most probable cause
of this trouble is a reversal in the stator and rotor connec-
tions. The stationary plates should be connected to the
grid terminal of the tuning transformer, and the rotary
plates to the grid-return terminal. Shielding is also
effective in removing this difficulty. |

Misalinement of Multiple Condensers.—A large num-
ber of present-day receiving sets are equipped with single-
control dials. In some of these there is no form of com-
pensation for discrepancies in the tuning circuits, and all
adjustments must be made at the main condensers. In
other cases, vernier condensers are employed to bring the
circuits to resonance when minor discrepancies in tuning
develop.

When radio reception is very weak and it is positively
known that the batteries, or power units, tubes, trans-
formers, by-pass condensers, and their connections check
O. K., the trouble may generally be ascribed to a misaline-
ment of the tuning condensers. Wide discrepancies in the.
positions of the condenser plates can be determined by
inspection. Minor discrepancies, however, can be deter-
mined only by suitable electrical tests.

When there is no provision for changing the position
of either the rotary or stationary plates, the only remedy
is the replacement of the entire condenser assembly.
Condenser units with adjustable rotors or stators can be
brought to resonance as follows:

AND SERVICING 135

Set the oscillator, Fig. 4, in operation with the dial set
near one end of its scale. Place the receiving set also in
operation with the aerial and ground disconnected. Tie
one end of a 20-foot insulated wire around the grid coil of
the detector tube and place the other end near the oscillator.
Remove all the radio-frequency tubes and tune the receiver
to maximum signal, setting the vernier controls, if used,
in their mid-positions. The position of the dial for maxi-
mum signal is marked in a convenient location.

Remove the pick-up wire from the tuning coil of the
detector tube, place it around the tuning coil of the last
radio-frequency tube, and replace the tube in its socket.
Tune the set as before, noting whether the position for
maximum signal corresponds with that previously obtained.
If there is a discrepancy, see whether it can be corrected
with the vernier dial, if used. Otherwise, shift the tuning-
condenser rotor or stator, whichever is adjustable, until the
positions for maximum signal intensity correspond. Pro-
ceed in the same manner with the remaining radio-fre-
quency circuits, working backwards from the detector
tube to the first radio-frequency amplifier tube.

Then set the oscillator at the other end of its scale and
repeat the foregoing tests and adjustments. Generally,
when the set has been adjusted at one frequency it will be
found satisfactory on all other frequencies, but it is well
to check it and make readjustments when necessary.

TESTING FIXED CONDENSERS

The simplest way to test fixed by-pass or filter con-
densers is by the charge and discharge method. Discon-
nect the leads from the terminals of the condenser and
connect the condenser for a brief interval across a suitable
source of d.-c. potential, such as a B battery or the output
of a power unit. Disconnect the condenser from the

136 SRADIO SRECEIVERS

power source and connect a piece of wire across its termi-
nals. If the condenser is in good condition a discharge
spark should take place the instant the condenser terminals
are shorted. If no discharge spark or a sharp crack takes
place, the condenser is open, short-circuited, or leaky, and
should be replaced.

ADJUSTMENT OF NEUTRALIZING CON DENSERS

In practically all radio receiving sets some form of
balancing is employed to reduce the tendency to self-
oscillation. When this adjustment is unbalanced, the
receiver has a tendency to oscillate at practically all
settings of the station selector dials. Before attempting
to balance such a set it is well to check the filament plate
and grid voltages, examine the grid circuits for opens, and
test the tubes. The trouble is manifested by poor-quality
signals and whistling and howling in the loud speaker.

There are two common methods of stopping oscillations;
namely, by the use of grid resistors and by the neutrodyne
system. Where grid resistors are used, the procedure is
quite simple. Remove the resistor from its clips and test
it with the unit shown in Fig. 3. If it is open, short-
circuited, or of the wrong value, replace it with one of the
correct resistance value.

In sets employing the neutrodyne system, small adjust-
able condensers are used to effect balancing. Should these
get out of adjustment, they may be readily readjusted as
follows: Procure a tube similar to those used in the
radio-frequency stages and saw off one of the filament
prongs. Place the receiving set in operation with the
aerial and ground connected, and the oscillator, Fig. 4,
near the aerial wire. Tune the oscillator and set to a
low reading on the dials, adjusting the set to maximum
loudness. Remove the first radio-frequency tube from

AND SERVICING 137

the set and insert in its place the special tube. Now adjust
the neutralizing condenser until the signal is minimum or
disappears entirely. The adjustment of the neutralizing
condenser is quite critical and should be done with care.
When the first stage has thus been neutralized, remove
the special tube and reinsert the good one. The remaining
condensers are adjusted in the same manner. The adjust-
ment is checked with the oscillator and set tuned to a
high reading on their respective dials.

SERVICING OF POWER UNITS

Determining Whether Power Unit Is At Fault.—When
radio reception is unsatisfactory and it is suspected that
the B power unit is at fault, it is advisable first to check up
on the other accessories, such as tubes, A battery, C battery,
aerlal and ground connections, and the loud speaker. If
all these seem to be in good condition, it is well to substi-
tute a set of B batteries for the power unit and note the
difference in operation. If this test shows that the B unit
is at fault, the first thing to do is to make sure that the
socket power is on and to try a tested rectifier tube in
place of the one that is in the power unit. If the new tube
improves the operation of the set, obviously, the trouble
has been corrected.

Testing the Power Unit.—If the new tube does not
improve the operation, the power unit should be tested
for opens, short circuits, and grounds. With a circuit
diagram at hand, proceed to test the continuity of the
circuits. The continuity tester shown in Fig. 2 may be
used. The power should, of course, be turned off.’ The
test would include the resistors and their shunting con-
densers in the output of the unit; the filter condensers;
the filter choke coils; the windings of the power transformer,
and all the connections and wiring in the unit.

138 ‘RADIO SRECEIVERS

An experienced serviceman has usually a few short cuts
for locating trouble in power units. He knows from
experience the points at which trouble is most likely to
develop, and acts accordingly. With a steel Screw-driver
he can determine whether there is any current flowing in
the chokes by noting the magnetic pull on the screwdriver.
By short-circuiting the chokes and noting the effect on the
receiver, he can determine whether the filter condensers
are defective. There are many other practical methods
of quickly determining the causes of certain troubles
which the observant serviceman will gather in the course
of his work. Such tests presuppose a familiarity with the
equipment and should not be attempted by beginners.

SERVICING OF RADIO SPEAKERS

Horn-Type Speakers.—Before proceeding to examine
any loud speaker for defects, it is well to test the output of
the receiving set with a speaker that is known to be in good
condition. This will help to confine the trouble to the
proper source. When it has been determined that the
speaker is at fault, the first thing to do is to test the con-
tinuity of the windings. If these are open or short-cir-
culted, the speaker will not operate. Sometimes the
defect is in the cord or in the connections to the terminals
and may be determined by inspection or by tests.

A rattling sound is generally produced by the dia-
phragm or armature, if a direct drive is used, touching the
magnet poles. This can usually be remedied on an adjust-
able unit by moving the coils away from the diaphragm.
If the diaphragm is defective it should be replaced. In
indirectly driven units the armature is connected to the
diaphragm by means of a short pin. If the armature strikes —
the pole pieces a rattling sound will result. Some form
of compensation is usually provided to center the armature,

AND aS ER Vic NoG 139

but when this does not correct the fault the adjustment
should be loosened and a thin piece of card board place
on each side of the armature between the pole pieces; then
the pin between the armature and diaphragm should be
loosened until it assumes a normal position. Fastening
the diaphragm to the connecting pin in this unstrained
position wil! usually eliminate the rattle.

The loud speaker may also develop other troubles, as, for
example, weakened magnets, iron or dust particles on the
pole pieces, worn or defective rubber gaskets, if used, loose
diaphragm, etc. The same for any one of these defects
is quite obvious.

Cone-Type Speakers. —In cone-type radio speakers
trouble may develop in the electromagnetic operating
unit, in the cone, or in the mechanical connection between
the unit and the cone. ‘The troubles previously considered
in connection with the operating unit of the horn-type
speaker apply also to the operating unit of the cone-type
speaker. Open windings, armature not centered, dust
particles between the armature and the pole pieces, are
troubles common to both the horn- and cone-type speakers.

In some cases the cone may be improperly alined or
adjusted, causing a strain to be placed on the driving pin.
Poor reproduction is the result, and an inspection of the
drive pin may indicate a slight torque or twist. This is
most likely to happen when replacing a cone. Loose screws
or nuts in the motor mechanism may also cause a rattle
when the speaker is in operation. The proper remedy is to
tighten all the nuts and screws.

Cone-type speaker troubles may be briefly summarized
as follows:

No Signals.—No output from receiver; defective wind-
ings; defective cord; loose or broken connections; drive
pin not connected.

140 SRADIO SRECEIVERS

Weak Signals.—Weak receiver output; dirt interfering
with armature action; drive pin improperly connected ;
improperly alined cone; weak magnet.

Distorted or Noisy Signals.—Distorted output from
receiver; improperly adjusted cone; loose screws or nuts
in the assembly; armature striking pole pieces ; excessive
pressure on drive pin.

Howling.—Microphonic tubes; speaker too near the
receiver.

Electrodynamic Speakers.—The electrodynamic speaker
is supposed to be one of the truest reproducers of speech
and music. Because of its cost, the owner expects first-
class results, and justly so. Like all other types of speakers,
however, the dynamic speaker is subject to troubles and
defects which are just as annoying as those of other
speakers. There are several types of these speakers,
operating on similar principles, but with sufficient modifica-
tions to develop their own characteristic troubles. The
symptoms and causes given here apply in a general way to
all types.

No Signals.—No output from receiving set or amplifier ;
defective connections; defective field coil; defective cone
coil.

Weak Sztgnals.—Weak output from receiver or power
amplifier; weak field.

Poor-Quality Signals.—Poor output from receiver ; cone
or reproducer unit not centered properly; cone coil or the
connecting wires loose on cone.

WEN) EeRevi LOL NLG I4I

CONCLUSION

The foregoing are just a few of the service problems
encountered in every-day work. The classification and
solution of these problems has been aided materially by
referring to the service notes of a few representative radio
manufacturers. The instruction was compiled to help the
beginner to solve some of the more common service prob-
lems and it is hoped that it will also be of benefit to the
more mature serviceman.

INDEX

A
Adapters, 122
Amplifier, Audio-frequency, Impedance-
coupled, 73
Audio - frequency,
coupled, 70
Amplifiers, Audio-frequency, 70
Methods of stabilizing radio-fre-
quency, 42
Power, 79
Radio-frequency, 19
Audio - frequency amplifier, Trans.
former-resistance coupled, 77
-frequency amplifiers, 70

Transformer -

F B
Bell ringers, 117
Bias detector, 7
Binding posts, 121
Body capacity, 134

Cc
Circuit for reception of undamped
waves, 12
Receiving, 2
Circuits, Audio-frequency, 46
Classification of receiving-set, 104
Open, Causes of, 106
Short, Causes of, 106
Commutators, 117
Crystal detectors, 6

D

Detection, Interception and, 13%
Detector, Bias, 7
Diode, 6
Function of, 5
Using of grid condenser and grid
leak, 8
Detectors, Crystal, 6
Vacuum-tube, 6
Dial switch, 121

F

Failure, Relation between length of
service and, 110
Failures in receiving sets, 105

G
Grid resistors, 42

H

Headset, Radio, 99
High-resistance connection, 107

I

Impedance - coupled audio - frequency
amplifier, 73

Interception and detection, 13

Interference, Outside, 113

L
Loud speaker, 100

M
Modulated oscillator, 131

N
Neutrodyne receiver, 34

O
Oscillator, Modulated, 131

Pp
Portable tool kit, 108
Power amplifier and B and C eliminator,
91
amplifier, Definition of, 81
amplifiers, 79
-line induction, 114
plate supply, 79
Precipitators, 117

R
Radio-frequency amplifiers, 19
-frequency amplifiers, Methods of
stabilizing, 42

Radio-frequency with feed-back, One-
stage tuned, 22
headset, 99
receivers, 1
receivers, Servicing of, 104
receivers, Theory of operation, 1
Reception from a greater distance, How
effected, 13
of undamped waves, 10
Peculiarities of short-wave, 56
Short-wave, 56
Receiver, 300-19,000-meter commercial,

15

Neutrodyne, 34

Short-wave, with interchangeable
coils, 57

Single-circuit, 14
Single-side-band, 65
Standard neutrodyne, 40
Superheterodyne, 47
Two-stage tuned radio-frequency, 25
Untuned-transformer coupled, 19
Watch-case, 98
Receivers, Failures of, 105
Radio, 1
Reflex, 44
Regenerative, 14
Short-wave, 56
Telephone, 96
Theory of operation of, 1
with a.-c. tubes, 94
with figure-8 coils, 31
Receiving circuit, 2
sets, 104
sets, Testing of, 118
Reflex receivers, 44
Regeneration by tuned-plate method, 18
Regenerative receivers, 14
Resistance measurement, 131
Root, Mean square, 83

S)

Service-shop equipment, 108
Servicing of power units, 137

of radio receivers, 104

of radio speakers, 138
Shielding, 43
Shorted plates, 133
Short-wave receivers, 56

-wave throttle tuner, 62
Single-side-band receiver, 65

il INDEX

Sound reproducers, 96
Sparking apparatus, 115.
Speaker, Cone-type, 101

Electro-dynamic, with power ampli-

fier, 101

Horn-type, 100

Loud, 100
Speakers, 100

Servicing of radio, 138
Superheterodyne receiver, 47
Switch, a.-c., d.-c., 121

Dial, 121

T
Telephone receivers, 96
Tester, Continuity, 130
Purpose of, 118
Weston a.-c,. and d.-c., 118
Testing a.-c. operated sets, 127
battery-operated sets, 122
fixed condensers, 135
the power unit, 137
Throttle tuner, 62
Tool kit, Portable, 108
Transformer-coupled
amplifier, 70
“Tesistance coupled audio-frequency
amplifier, 77
Transformers, Construction of radio-
frequency, 29
Transmitting station,
radio-telegraph, 3
station, Non-directive, it
Trouble in accessories, 111
Probable sources of, 111
Troubles and adjustments, Specific, 130
Variable-condenser, 133
Tube, UX-201-A, 79

audio-frequency

Damped-wave

UX-210, 88
UX-216-B, 86
UX-281, 86
U
Undamped waves, Reception of, 10
V

Volt-milliammeter, d.-c., 119
Voltmeter, a.-c., 118

WwW
Wave trains, Illustration of, 4
Waves, Reception of undamped, 10

— el
—_—) ee

a